CI: custom runner-base image, Node 22, pip caching

- infra/Dockerfile.runner-base: new Ubuntu 24.04 image with Python 3.12
  and Node 22 LTS pre-installed, tagged py3.12-node22. Eliminates the
  ~3-4 min deadsnakes PPA install that ran on every test job.
- infra/runner-compose.yml: update ubuntu-latest label to use
  runner-base:py3.12-node22 instead of node:20-bullseye.
- .forgejo/workflows/ci.yml: remove Python 3.12 install steps (now in
  base image), add actions/cache for pip keyed on pyproject.toml hash,
  bump node-version 20 → 22.
- Dockerfile: bump frontend build stage node:20-alpine → node:22-alpine.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/Dockerfile.runner-base

@@ -0,0 +1,24 @@
+# Runner base image for Forgejo act_runner job containers.
+# Pre-installs Python 3.12 and Node 20 so the test job skips the
+# ~3-4 minute deadsnakes PPA install on every run.
+#
+# Build and push (one-time setup, re-run if this file changes):
+#   docker build -f infra/Dockerfile.runner-base \
+#     -t git.fabledsword.com/bvandeusen/runner-base:py3.12-node22 .
+#   docker push git.fabledsword.com/bvandeusen/runner-base:py3.12-node22
+#
+# Then redeploy the act_runner stack in Portainer to pick up the new label.
+
+FROM ubuntu:24.04
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+# Python 3.12 ships in Ubuntu 24.04 main repos — no PPA needed.
+RUN apt-get update -qq && \
+    apt-get install -y -qq \
+      python3.12 python3.12-dev python3.12-venv python3-pip \
+      git curl ca-certificates gnupg && \
+    # Node 22 LTS via NodeSource
+    curl -fsSL https://deb.nodesource.com/setup_22.x | bash - && \
+    apt-get install -y -qq nodejs && \
+    apt-get clean && rm -rf /var/lib/apt/lists/*

infra/runner-compose.yml

@@ -13,7 +13,7 @@ services:
       - GITEA_RUNNER_REGISTRATION_TOKEN=${RUNNER_TOKEN}
       - GITEA_RUNNER_NAME=swarm-runner
       # Maps the runs-on: ubuntu-latest label used in workflow files to a Docker image.
-      - GITEA_RUNNER_LABELS=ubuntu-latest:docker://node:20-bullseye
+      - GITEA_RUNNER_LABELS=ubuntu-latest:docker://git.fabledsword.com/bvandeusen/runner-base:py3.12-node22
       - CONFIG_FILE=/config/config.yml
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock