Speed up repeated runtime image builds

.github/workflows/pr.yml

@@ -38,6 +38,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - run: bash scripts/docker-build-cache.test.sh
+        name: Validate Docker build cache configuration
       - uses: actions/setup-go@v5
         with:
           go-version: "^1.24"
@@ -53,6 +55,7 @@ jobs:
             .docker/druid-install-command.sh
 
   prerelease:
+    if: github.event.pull_request.head.repo.full_name == github.repository
     runs-on: ubuntu-latest
     needs: [unit-tests, integration-tests, validate-api, build]
     outputs:

Dockerfile

@@ -1,15 +1,25 @@
+# syntax=docker/dockerfile:1
+
 FROM golang:bullseye AS builder
 
 ARG VERSION=docker
 
-COPY . .
-COPY .docker/entrypoint.sh /entrypoint.sh
+WORKDIR /src
 
-WORKDIR /go
+COPY go.mod go.sum ./
+RUN --mount=type=cache,id=druid-go-mod,target=/go/pkg/mod,sharing=locked \
+    --mount=type=cache,id=druid-go-build,target=/root/.cache/go-build,sharing=locked \
+    go mod download && \
+    go install github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@v2.5.1
 
 ENV VERSION=${VERSION}
 
-RUN make build
+COPY . .
+COPY .docker/entrypoint.sh /entrypoint.sh
+
+RUN --mount=type=cache,id=druid-go-mod,target=/go/pkg/mod,sharing=locked \
+    --mount=type=cache,id=druid-go-build,target=/root/.cache/go-build,sharing=locked \
+    make build
 
 # The binaries are in ./bin/ directory after build
 
@@ -34,7 +44,7 @@ RUN ARCH=$(uname -m) && \
     chmod +x /usr/bin/yq
 
 # Copy only the built binaries and entrypoint from builder
-COPY --from=builder /go/bin/druid* /usr/bin/
+COPY --from=builder /src/bin/druid* /usr/bin/
 COPY --from=builder /entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
 

scripts/docker-build-cache.test.sh

@@ -0,0 +1,46 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+DOCKERFILE="$SCRIPT_DIR/../Dockerfile"
+
+python3 - "$DOCKERFILE" <<'PY'
+from pathlib import Path
+import sys
+
+dockerfile = Path(sys.argv[1]).read_text(encoding="utf-8")
+
+if not dockerfile.startswith("# syntax=docker/dockerfile:1\n"):
+    raise SystemExit("Dockerfile does not opt into BuildKit cache mounts")
+if "WORKDIR /src" not in dockerfile:
+    raise SystemExit("Go module build must run outside GOPATH in /src")
+if "COPY --from=builder /src/bin/druid* /usr/bin/" not in dockerfile:
+    raise SystemExit("Runtime stage does not copy binaries from /src")
+
+dependency_copy = dockerfile.find("COPY go.mod go.sum ./")
+source_copy = dockerfile.find("COPY . .")
+module_download = dockerfile.find("go mod download")
+generator_install = dockerfile.find(
+    "go install github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@v2.5.1"
+)
+version_env = dockerfile.find("ENV VERSION=${VERSION}")
+
+if min(dependency_copy, source_copy, module_download, generator_install, version_env) < 0:
+    raise SystemExit("Dockerfile is missing the dependency-first Go build layer")
+if not dependency_copy < module_download < source_copy:
+    raise SystemExit("Go modules are not prepared before the full source copy")
+if not generator_install < source_copy:
+    raise SystemExit("oapi-codegen is reinstalled after every source change")
+if not generator_install < version_env < source_copy:
+    raise SystemExit("VERSION invalidates the stable dependency/tool layer")
+
+source_build = dockerfile[source_copy:]
+for target in ("target=/go/pkg/mod", "target=/root/.cache/go-build"):
+    if target not in source_build:
+        raise SystemExit(f"Source build is missing cache mount {target}")
+if "make build" not in source_build:
+    raise SystemExit("Source build no longer runs the required make build target")
+PY
+
+echo "Docker build keeps required work while reusing Go dependency and compiler caches."