ci: publish canary container images to ghcr

.github/workflows/ci.yml

@@ -13,7 +13,14 @@ on:
 
 concurrency:
   group: ci-${{ github.ref }}
-  cancel-in-progress: true
+  # Superseded PR pushes are worth cancelling; main is not. A cancelled main run takes
+  # publish-canary down with it (run-level cancellation ignores job-level concurrency),
+  # and that commit's immutable sha- tag would then never exist.
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
+
+# Least privilege by default: only publish-canary needs a write scope, and it asks for it itself.
+permissions:
+  contents: read
 
 env:
   CARGO_TERM_COLOR: always
@@ -81,3 +88,82 @@ jobs:
       - name: Build the SPA and the SDK
         run: just web-build
       - run: just e2e
+
+  # Canary image publish. Lives in this workflow, not a separate one, so `needs` gates it on the
+  # same green run that just tested the commit: a red main never produces a canary tag, and there
+  # is no workflow_run indirection re-deriving the SHA from an event payload.
+  #
+  #   ghcr.io/<owner>/walgit:canary    moving tag, always the newest green main
+  #   ghcr.io/<owner>/walgit:sha-<40>  immutable, what a rollback or a bug report pins to
+  #
+  # Re-running a run on the same commit is safe, and it does NOT touch an existing sha- tag: the
+  # build is not bit-reproducible, so republishing that tag would repoint what a rollback pinned to
+  # at a digest nobody chose. A re-run moves `canary` only; the sha- tag is written once, ever.
+  publish-canary:
+    name: publish canary image
+    needs: [build-test, e2e]
+    if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      # The only write scope this workflow needs; GITHUB_TOKEN carries nothing else.
+      packages: write
+    steps:
+      # This job holds a token that can write packages, so its actions are pinned to commit SHAs:
+      # a moving tag would let an upstream compromise reach the registry. The other jobs read only.
+      - uses: actions/checkout@11d5960a326750d5838078e36cf38b85af677262 # v4.4.0
+        with:
+          # Nothing here runs git again; leaving the token in .git/config only widens the blast radius.
+          persist-credentials: false
+      - uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3.12.0
+
+      - uses: docker/login-action@c94ce9fb468520275223c153574b00df6fe4bcc9 # v3.7.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # GHCR rejects an uppercase path, and github.repository carries the owner's real casing.
+      - name: Resolve the image name
+        id: image
+        run: echo "name=ghcr.io/${GITHUB_REPOSITORY,,}" >> "$GITHUB_OUTPUT"
+
+      # "Immutable" has to mean it: if this commit already has a sha- tag, leave it at the digest
+      # it already points to and let this run move `canary` alone.
+      - name: Decide whether the rollback tag is still unwritten
+        id: rollback
+        run: |
+          if docker buildx imagetools inspect "${{ steps.image.outputs.name }}:sha-${GITHUB_SHA}" >/dev/null 2>&1; then
+            echo "unwritten=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "unwritten=true" >> "$GITHUB_OUTPUT"
+          fi
+
+      # format=long, not short: a seven-character prefix is a namespace that collides, and the
+      # collision would silently move an older commit's immutable tag onto a newer image. Both tags
+      # and the OCI source/revision labels come from one place rather than hand-built strings.
+      - name: Derive tags and labels
+        id: meta
+        uses: docker/metadata-action@c299e40c65443455700f0fdfc63efafe5b349051 # v5.10.0
+        with:
+          images: ${{ steps.image.outputs.name }}
+          tags: |
+            type=raw,value=canary
+            type=sha,format=long,prefix=sha-,enable=${{ steps.rollback.outputs.unwritten }}
+
+      - name: Build and push
+        uses: docker/build-push-action@10e90e3645eae34f1e60eeb005ba3a3d33f178e8 # v6.19.2
+        with:
+          context: .
+          file: Containerfile
+          platforms: linux/amd64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          # The build stage never copies .git, so this ARG is the binary's only build identity:
+          # without it walgit-server/build.rs falls back to "dev" and every image reports the same
+          # version on /healthz. Full 40 chars: build.rs takes the value verbatim.
+          build-args: |
+            WALGIT_BUILD_SHA=${{ github.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

README.md

@@ -127,7 +127,8 @@ open https://walgit.localhost:8080/
 
 * `walgit.standalone.toml` — the one-machine shape (self-signed TLS, rustfs, every role). Start here.
 * `walgit.example.toml` — every key with its default and a comment.
-* `Containerfile`, `flake.nix` — an OCI image and a Nix package/devshell.
+* `Containerfile`, `flake.nix` — an OCI image and a Nix package/devshell. Every green push to `main` publishes
+  that image to `ghcr.io/<owner>/walgit` as `canary` (moving) and `sha-<40>` (immutable: what a rollback pins to).
 * `deploy/nginx.conf.example` — an optional nginx in front: public TLS, one `auth_request` per credential, and
   **byte offload**: walgit answers bundle/LFS downloads with `X-Accel-Redirect` and nginx streams + caches the
   object from the bucket itself (S3 presigned or GCS with walgit's bearer). The file documents the contract.