build: move releases to CI and trim the Makefile

Merging a pull request into main is now the whole release. A Gitea
Actions workflow derives the CalVer tag, builds the image and pushes
it with :latest, so nothing is built locally any more.

That made image/push/release redundant, and with them the .release-tag
state file and the main-branch guard — the workflow only runs on main,
which is protected, so the guard had nothing left to catch. The digest
verification went too: it guarded a `make -j` race between image and
push that cannot happen in a single CI job.

seed, icons and vendor ran a few times a year and are written out in
the README instead. Makefile: 151 lines to 71.

Docs referenced the removed targets in sixteen places, including a
CONTRIBUTING note claiming the branch check "has to be local".

.gitea/workflows/release.yaml

@@ -0,0 +1,58 @@
+name: release
+on:
+  push:
+    branches: [main]
+
+# ponytail: only because Traefik still serves its default self-signed cert for
+# git.kessinen.com. Remove once the LE-DNS01-cloudflare runbook has been run.
+env:
+  GIT_SSL_NO_VERIFY: "true"
+  REGISTRY: git.kessinen.com
+  IMAGE: git.kessinen.com/kessinen/foodster
+
+jobs:
+  image:
+    runs-on: ubuntu-latest
+    steps:
+      # Full history and tags: the release number is derived by counting the
+      # tags already cut today.
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      # The job container is node:22-bookworm and has no docker client. The
+      # static binary is one file; installing docker.io would pull a daemon
+      # that is never used, since the build runs against the host's.
+      - name: Install the docker client
+        run: |
+          curl -fsSL https://download.docker.com/linux/static/stable/x86_64/docker-27.3.1.tgz \
+            | tar xz --strip-components=1 -C /usr/local/bin docker/docker
+          docker version --format '{{.Client.Version}}'
+
+      - name: Work out the release tag
+        id: rel
+        run: |
+          day=$(date +%Y%m%d)
+          tag="v$day-$(( $(git tag -l "v$day-*" | wc -l) + 1 ))"
+          echo "tag=$tag" >> "$GITHUB_OUTPUT"
+          echo "==> $tag"
+
+      - name: Tag the commit
+        run: |
+          git tag "${{ steps.rel.outputs.tag }}"
+          git push origin "${{ steps.rel.outputs.tag }}"
+
+      - name: Log in to the registry
+        run: |
+          echo "${{ secrets.GITEA_TOKEN }}" \
+            | docker login "$REGISTRY" -u "${{ gitea.actor }}" --password-stdin
+
+      - name: Build and push
+        run: |
+          tag="${{ steps.rel.outputs.tag }}"
+          docker build --platform linux/amd64 --build-arg VERSION="$tag" \
+            -f Containerfile \
+            -t "$IMAGE:$tag" -t "$IMAGE:latest" .
+          docker push "$IMAGE:$tag"
+          docker push "$IMAGE:latest"
+          echo "pushed $IMAGE:$tag and :latest - pull it in dockge when ready"