CI: skip unchanged base image and add registry build cache

Two build-speed changes to the Gitea Actions deploy workflow.

(#1) Build the base image only when it changes. The base layers' only
repo input is server-ce/Dockerfile-base, so the prepare step hashes that
file and the base is tagged verso-base:base-<hash>; the app builds FROM
that exact tag. If a base with the current hash already exists in the
registry, the heavy base build (apt ~111s, TeX Live ~51s, Quarto, plus
its ~49s export/push) is skipped entirely — which is every commit that
doesn't touch Dockerfile-base.

(#2) Import/export a registry-backed layer cache (verso-cache:base and
verso-cache:app, mode=max) on both builds. Unchanged layers are reused
instead of rebuilt: yarn install is skipped when package.json is
unchanged, and only the web compile re-runs on a frontend source change.
No new cluster resources — the cache lives as extra tags in the same
in-cluster registry.

First run after this is still a full build (populates the caches and the
hash-tagged base); subsequent commits should be substantially faster.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/deploy-verso.yml

@@ -37,7 +37,23 @@ jobs:
                     args:
                       - |
                         set -eux
+                        REG=registry.git.svc.cluster.local:5000
                         git clone --depth 1 https://git.alocoq.fr/alois/verso.git /workspace/repo
+
+                        # (#1) Build the base image only when it actually changes.
+                        # The base layers' only repo input is Dockerfile-base, so
+                        # we key on a content hash of that file: the base is tagged
+                        # verso-base:base-<hash> and the app builds FROM that exact
+                        # tag. If a base with this hash is already in the registry,
+                        # the heavy base build (apt, TeX Live, Quarto) is skipped.
+                        BTAG=$(sha256sum /workspace/repo/server-ce/Dockerfile-base | cut -c1-16)
+                        printf '%s' "$BTAG" > /workspace/base_tag
+                        if wget -qO- "http://$REG/v2/verso-base/tags/list" 2>/dev/null | grep -q "\"base-$BTAG\""; then
+                          echo "Base image base-$BTAG already present - skipping base build"
+                        else
+                          touch /workspace/build-base
+                          echo "Base image base-$BTAG not found - base will be built"
+                        fi
                     volumeMounts:
                       - name: workspace
                         mountPath: /workspace
@@ -54,30 +70,47 @@ jobs:
 
                         # Push to the in-cluster registry (plain HTTP) to bypass
                         # the Traefik ingress, whose read timeout was killing the
-                        # multi-GB TeX Live layer upload mid-stream. The base
-                        # image is pulled back in for the second build, so the
-                        # registry must be marked insecure for both push and pull.
-                        # Write buildkitd.toml inside the container (no extra
-                        # k8s resources needed) so the second build's pull/resolve
-                        # treats the registry as http.
+                        # multi-GB TeX Live layer upload mid-stream. Mark the
+                        # registry http+insecure so both push and the base pull
+                        # for the app build treat it as plain HTTP. Written inside
+                        # the container so no extra k8s resources are needed.
                         REG=registry.git.svc.cluster.local:5000
 
                         mkdir -p /etc/buildkit
                         printf '[registry."%s"]\n  http = true\n  insecure = true\n' "$REG" > /etc/buildkit/buildkitd.toml
 
-                        buildctl-daemonless.sh build \
-                          --frontend=dockerfile.v0 \
-                          --local context=/workspace/repo \
-                          --local dockerfile=/workspace/repo/server-ce \
-                          --opt filename=Dockerfile-base \
-                          --output type=image,name=$REG/verso-base:latest,push=true,registry.insecure=true
+                        BTAG=$(cat /workspace/base_tag)
+                        BASE_REF="$REG/verso-base:base-$BTAG"
 
+                        # (#1) Base build, only when prepare flagged it changed.
+                        # (#2) Import/export a registry layer cache so that, when
+                        # the base does change, unchanged layers (e.g. apt) are
+                        # still reused instead of rebuilt from scratch.
+                        if [ -f /workspace/build-base ]; then
+                          buildctl-daemonless.sh build \
+                            --frontend=dockerfile.v0 \
+                            --local context=/workspace/repo \
+                            --local dockerfile=/workspace/repo/server-ce \
+                            --opt filename=Dockerfile-base \
+                            --import-cache type=registry,ref=$REG/verso-cache:base \
+                            --export-cache type=registry,ref=$REG/verso-cache:base,mode=max \
+                            --output type=image,name=$BASE_REF,push=true,registry.insecure=true
+                        else
+                          echo "Reusing existing base image $BASE_REF"
+                        fi
+
+                        # App image, built FROM the content-pinned base tag.
+                        # (#2) The registry cache lets yarn install be skipped when
+                        # package.json is unchanged; the web build only re-runs
+                        # when the frontend source actually changes.
                         buildctl-daemonless.sh build \
                           --frontend=dockerfile.v0 \
                           --local context=/workspace/repo \
                           --local dockerfile=/workspace/repo/server-ce \
                           --opt filename=Dockerfile \
-                          --opt build-arg:OVERLEAF_BASE_TAG=$REG/verso-base:latest \
+                          --opt build-arg:OVERLEAF_BASE_TAG=$BASE_REF \
+                          --import-cache type=registry,ref=$REG/verso-cache:app \
+                          --export-cache type=registry,ref=$REG/verso-cache:app,mode=max \
                           --output type=image,name=$REG/verso:latest,push=true,registry.insecure=true
                     volumeMounts:
                       - name: workspace