ci: pin the build clock to the commit, so an unchanged refresh publishes nothing

Milestone 362 step 1, closing #3265's root cause.

The weekly base refresh rewrote all three `:latest` tags on 2026-08-30 with
nothing changed in any of them. Not a cache miss — run 4934's log shows every
content step CACHED and both bases resolved to unchanged pinned digests.
buildkit stamps the image config with the wall clock of the build, so
identical layers get republished under a new config blob and therefore a new
manifest digest.

The cost is not storage, it is meaning: `:latest` moved on a calendar, so a
digest change stopped being evidence that anything was different. That is the
one thing a digest is any use for, and it is load-bearing here — the reuse
check, the `:c-<sha>` rollback story and any future redeploy signal all rest
on it.

SOURCE_DATE_EPOCH normalises `created` and the history timestamps, so the same
source produces the same config bytes and the same digest, and pushing it is a
registry no-op.

The value is routed through artifacts.sh's existing `newest()` rather than
taken from git separately. `revision`, `version` and now `epoch` are three
fields of ONE lookup, so they cannot drift into naming different commits — a
divergence that would stamp an image reproducibly against one commit while it
reported being another, with both values looking perfectly well-formed. Note
#3127 §2 is the record of what a second clock costs; this adds a view, not a
clock.

Also corrected: the build step comment and ci-requirements.md both described
the churn as current behaviour with the fix as a "likely" future. They now
describe what the file does.

Tests pin the property the fix depends on, not the fix: epoch is the same
commit version names, in both renderings including the extension's unpadded
one, and it does not move between two calls on one checkout. A future
refactor that gave epoch its own `git log` would pass every other test in
that file.

Not yet verified end to end — proving it needs two consecutive refreshes to
land on the same digest, which is the next thing, and is the step #3265 exists
because nobody did last time.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TTjbZZ6JirCMSaJzQV1RhA

.forgejo/workflows/build.yml

@@ -638,6 +638,14 @@ jobs:
           # adds no variability the reuse check would have to account for.
           echo "version=$(sh scripts/artifacts.sh version web)" >> "$GITHUB_OUTPUT"
 
+          # The build clock, pinned to the same commit (#3265). Without it
+          # buildkit stamps the image config with the wall clock of the build,
+          # so identical layers republish under a new config blob and the
+          # channel tag gets a new manifest digest for no reason. Derived from
+          # `newest()` like revision and version, so all three name one commit
+          # and cannot drift apart.
+          echo "epoch=$(sh scripts/artifacts.sh epoch web)" >> "$GITHUB_OUTPUT"
+
           # The moving tag for this channel. Which tag we ask IS the channel —
           # that is why the revision needs no -main/-dev qualifier any more.
           if [ "$CHANNEL" = "main" ]; then T=latest; else T=dev; fi
@@ -764,6 +772,12 @@ jobs:
 
       - name: Build and push web image
         if: steps.reuse.outputs.hit != 'true'
+        # Read by buildx out of the ENVIRONMENT, not passed as a build-arg —
+        # it normalises the image config's `created` field and the history
+        # timestamps rather than being consumed by the Dockerfile. See #3265
+        # and the reuse step's `epoch` output.
+        env:
+          SOURCE_DATE_EPOCH: ${{ steps.reuse.outputs.epoch }}
         uses: docker/build-push-action@v5
         with:
           context: .
@@ -776,20 +790,17 @@ jobs:
           # invalidates, and the image genuinely rebuilds.
           #
           # MEASURED on the first real fire, run 4934 (#3265): when the base
-          # did NOT move, the build is ~13s and every content step reports
-          # CACHED — but the channel tag STILL gets a new manifest digest.
-          # buildkit mints a fresh image config each run, so identical layers
-          # are republished under a new config blob. All three images moved
-          # that way on 2026-08-30 with nothing whatsoever changed in them.
+          # did NOT move, the build was ~13s with every content step CACHED —
+          # and the channel tag STILL got a new manifest digest, because
+          # buildkit stamps a fresh image config per run and republishes the
+          # identical layers under it. All three images moved that way on
+          # 2026-08-30 with nothing whatsoever changed in them.
           #
-          # So a refresh currently rewrites :latest every Sunday whether or
-          # not there is anything new in it, and :c-<sha> is handed a new
-          # manifest to diverge from on the same cadence. Layers are shared,
-          # so the storage cost is a config blob; the cost that matters is
-          # that a digest change no longer MEANS anything. Tracked in #3265 —
-          # the likely fix is a deterministic SOURCE_DATE_EPOCH, which would
-          # make "same source, same bytes" true and turn the no-op case into
-          # a genuine no-op.
+          # SOURCE_DATE_EPOCH (below) is the fix: pinned to the commit the
+          # content came from, the config is byte-identical across runs, so
+          # the manifest digest is too and the push is a registry no-op. A
+          # digest change means the content changed again, which is the only
+          # thing a digest is any use for.
           #
           # What `pull` does NOT catch either: a Debian package update inside
           # the `apt-get install` layer while the base tag itself stands
@@ -1097,6 +1108,14 @@ jobs:
           DERIVED=$(sh scripts/artifacts.sh revision ml)
           echo "revision=$DERIVED" >> "$GITHUB_OUTPUT"
 
+          # The build clock, pinned to the same commit (#3265). Without it
+          # buildkit stamps the image config with the wall clock of the build,
+          # so identical layers republish under a new config blob and the
+          # channel tag gets a new manifest digest for no reason. Derived from
+          # `newest()` like revision and version, so all three name one commit
+          # and cannot drift apart.
+          echo "epoch=$(sh scripts/artifacts.sh epoch ml)" >> "$GITHUB_OUTPUT"
+
           # The moving tag for this channel. Which tag we ask IS the channel —
           # that is why the revision needs no -main/-dev qualifier any more.
           if [ "$CHANNEL" = "main" ]; then T=latest; else T=dev; fi
@@ -1147,6 +1166,12 @@ jobs:
 
       - name: Build and push ml image
         if: steps.reuse.outputs.hit != 'true'
+        # Read by buildx out of the ENVIRONMENT, not passed as a build-arg —
+        # it normalises the image config's `created` field and the history
+        # timestamps rather than being consumed by the Dockerfile. See #3265
+        # and the reuse step's `epoch` output.
+        env:
+          SOURCE_DATE_EPOCH: ${{ steps.reuse.outputs.epoch }}
         uses: docker/build-push-action@v5
         with:
           context: .
@@ -1159,20 +1184,17 @@ jobs:
           # invalidates, and the image genuinely rebuilds.
           #
           # MEASURED on the first real fire, run 4934 (#3265): when the base
-          # did NOT move, the build is ~13s and every content step reports
-          # CACHED — but the channel tag STILL gets a new manifest digest.
-          # buildkit mints a fresh image config each run, so identical layers
-          # are republished under a new config blob. All three images moved
-          # that way on 2026-08-30 with nothing whatsoever changed in them.
+          # did NOT move, the build was ~13s with every content step CACHED —
+          # and the channel tag STILL got a new manifest digest, because
+          # buildkit stamps a fresh image config per run and republishes the
+          # identical layers under it. All three images moved that way on
+          # 2026-08-30 with nothing whatsoever changed in them.
           #
-          # So a refresh currently rewrites :latest every Sunday whether or
-          # not there is anything new in it, and :c-<sha> is handed a new
-          # manifest to diverge from on the same cadence. Layers are shared,
-          # so the storage cost is a config blob; the cost that matters is
-          # that a digest change no longer MEANS anything. Tracked in #3265 —
-          # the likely fix is a deterministic SOURCE_DATE_EPOCH, which would
-          # make "same source, same bytes" true and turn the no-op case into
-          # a genuine no-op.
+          # SOURCE_DATE_EPOCH (below) is the fix: pinned to the commit the
+          # content came from, the config is byte-identical across runs, so
+          # the manifest digest is too and the push is a registry no-op. A
+          # digest change means the content changed again, which is the only
+          # thing a digest is any use for.
           #
           # What `pull` does NOT catch either: a Debian package update inside
           # the `apt-get install` layer while the base tag itself stands
@@ -1466,6 +1488,14 @@ jobs:
           DERIVED=$(sh scripts/artifacts.sh revision agent)
           echo "revision=$DERIVED" >> "$GITHUB_OUTPUT"
 
+          # The build clock, pinned to the same commit (#3265). Without it
+          # buildkit stamps the image config with the wall clock of the build,
+          # so identical layers republish under a new config blob and the
+          # channel tag gets a new manifest digest for no reason. Derived from
+          # `newest()` like revision and version, so all three name one commit
+          # and cannot drift apart.
+          echo "epoch=$(sh scripts/artifacts.sh epoch agent)" >> "$GITHUB_OUTPUT"
+
           # The moving tag for this channel. Which tag we ask IS the channel —
           # that is why the revision needs no -main/-dev qualifier any more.
           if [ "$CHANNEL" = "main" ]; then T=latest; else T=dev; fi
@@ -1516,6 +1546,12 @@ jobs:
 
       - name: Build and push agent image
         if: steps.reuse.outputs.hit != 'true'
+        # Read by buildx out of the ENVIRONMENT, not passed as a build-arg —
+        # it normalises the image config's `created` field and the history
+        # timestamps rather than being consumed by the Dockerfile. See #3265
+        # and the reuse step's `epoch` output.
+        env:
+          SOURCE_DATE_EPOCH: ${{ steps.reuse.outputs.epoch }}
         uses: docker/build-push-action@v5
         with:
           context: agent
@@ -1528,20 +1564,17 @@ jobs:
           # invalidates, and the image genuinely rebuilds.
           #
           # MEASURED on the first real fire, run 4934 (#3265): when the base
-          # did NOT move, the build is ~13s and every content step reports
-          # CACHED — but the channel tag STILL gets a new manifest digest.
-          # buildkit mints a fresh image config each run, so identical layers
-          # are republished under a new config blob. All three images moved
-          # that way on 2026-08-30 with nothing whatsoever changed in them.
+          # did NOT move, the build was ~13s with every content step CACHED —
+          # and the channel tag STILL got a new manifest digest, because
+          # buildkit stamps a fresh image config per run and republishes the
+          # identical layers under it. All three images moved that way on
+          # 2026-08-30 with nothing whatsoever changed in them.
           #
-          # So a refresh currently rewrites :latest every Sunday whether or
-          # not there is anything new in it, and :c-<sha> is handed a new
-          # manifest to diverge from on the same cadence. Layers are shared,
-          # so the storage cost is a config blob; the cost that matters is
-          # that a digest change no longer MEANS anything. Tracked in #3265 —
-          # the likely fix is a deterministic SOURCE_DATE_EPOCH, which would
-          # make "same source, same bytes" true and turn the no-op case into
-          # a genuine no-op.
+          # SOURCE_DATE_EPOCH (below) is the fix: pinned to the commit the
+          # content came from, the config is byte-identical across runs, so
+          # the manifest digest is too and the push is a registry no-op. A
+          # digest change means the content changed again, which is the only
+          # thing a digest is any use for.
           #
           # What `pull` does NOT catch either: a Debian package update inside
           # the `apt-get install` layer while the base tag itself stands