ci: back Go compiles with the shared S3 build cache (#126)

Every pipeline run recompiled the whole tree from scratch; the shared gocache S3 bucket already holds those objects.

- Bootstrap the go-cache-plugin GOCACHEPROG in the test and pre-commit steps, keyed `ci-artifactapi`.
- Best-effort: a failed plugin fetch leaves GOCACHEPROG unset, and S3 errors degrade to cache misses, so an outage never fails a build.
- Run tests on the gobuilder image, which trusts the internal CA the S3 endpoint presents.

Container builds still compile uncached; plumbing the cache into the Dockerfile is a follow-up.

Reviewed-on: #126
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

.woodpecker/pre-commit.yaml

@@ -5,12 +5,26 @@ steps:
   - name: pre-commit
     image: git.unkin.net/unkin/almalinux9-gobuilder:20260606
     commands:
+      # Best-effort S3 build cache for the go vet hook: if the plugin cannot be
+      # fetched, GOCACHEPROG stays unset. S3 errors degrade to cache misses.
+      - "curl -fsSLo /tmp/gocacheprog https://git.unkin.net/unkin/go-cache-plugin/releases/download/v0.1.0/go-cache-plugin-linux-amd64 && echo '0eb23d06a0dcf395559cb6447864579c220e60148fdcd6a6f841c2b17169b9b8  /tmp/gocacheprog' | sha256sum -c - && chmod +x /tmp/gocacheprog && export GOCACHEPROG=\"/tmp/gocacheprog --cache-dir=/tmp/gocache\" || echo 'go build cache unavailable, compiling without it'"
       - uvx pre-commit run --all-files
     environment:
       # golib lives on Gitea; skip the public proxy/sum db.
       GOPRIVATE: git.unkin.net
+      GOCACHE_S3_BUCKET: gocache
+      # Explicit region skips a GetBucketLocation probe RGW handles poorly.
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: "https://s3.ceph.unkin.net"
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-artifactapi
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     backend_options:
       kubernetes:
+        serviceAccountName: default
         resources:
           requests:
             memory: 512Mi