fix(docker): a container's memory is its processes' own; file cache shown on its page (#6030)

The agent sent Docker's working set (usage less inactive_file), which still
counts recently used file cache. A file-heavy container with no limit read
191 GB while its processes held 2.2 GB, and the host's own figure, which
already leaves cache out, disagreed with it.

Agent 1.23.0 sends mem_used_bytes / mem_used_pct (anon + shmem, so
Postgres's shared buffers count; v1: rss + shmem) and mem_cache_bytes
apart. The new names keep the two meanings apart in history and in learned
normals: docker_013 renames the columns, clears the old values and adds
mem_cache_bytes. The Docker page, swarm by-node sums, Busiest Containers,
container detail, normals and the MCP tools all read the new figure. File
cache is shown only on the container's page and in get_container.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/docker/templates/docker/_usage.html

@@ -2,7 +2,8 @@
 
     Docker counts CPU per core (100% = one core busy), so CPU reads as cores,
     toned by its share of the host's CPUs when the host's core count is known.
-    Memory reads in bytes, against a limit only when one is actually set;
+    Memory is what its processes hold, file cache left out (#6030), and
+    reads in bytes, against a limit only when one is actually set;
     mem_pct is then a share of that limit, otherwise of the host's memory.
     Each is toned by usage()'s grade when it has one: within the container's
     own normal for the hour reads healthy, and says what is usual (#5742). -#}

plugins/docker/templates/docker/container_detail.html

@@ -78,6 +78,10 @@
       {%- if c.exit_code is not none and c.status == 'running' and c.restart_count %} <span class="tone-dim">· last exit {{ c.exit_code }}</span>{% endif %}</dd>
     {% endif %}
     <dt>Memory limit</dt><dd>{% if usage.mem_limit %}{{ usage.mem_limit|bytes }}{% else %}<span class="tone-dim">none set</span>{% endif %}</dd>
+    {#- Its memory figure is what its processes hold; the file cache charged to
+        it is shown here only, since the kernel gives it back when the host
+        needs it (#6030). #}
+    <dt>File cache</dt><dd>{% if c.mem_cache_bytes is not none %}{{ c.mem_cache_bytes|bytes }} <span class="tone-dim">files it read or wrote, kept in memory until the host needs it back; not counted in its memory</span>{% else %}<span class="tone-dim">—</span>{% endif %}</dd>
     {% if ports %}
     <dt>Ports</dt><dd class="mono">{% for p in ports %}{{ p.host_port }}:{{ p.container_port }}/{{ p.protocol }}{% if not loop.last %}, {% endif %}{% endfor %}</dd>
     {% endif %}