ci: move Go steps to gobuilder with S3 build cache

Switches the estate's older base images off in favor of the
shared gobuilder image and wires up the S3-backed Go build cache.

- Point build/test/lint-adjacent test/pre-commit/integration/release
  steps at artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
- Add GOCACHE_S3_* env and GOCACHEPROG=go-cache-plugin with an
  absolute --cache-dir to each Go-toolchain step
- Leave the dedicated golangci-lint step and the RPM upload step
  on their existing images

.woodpecker/build.yaml

@@ -3,7 +3,19 @@ when:
 
 steps:
   - name: build
-    image: golang:1.25
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
+    environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - make build
     backend_options:

.woodpecker/integration.yaml

@@ -5,16 +5,26 @@ when:
 # stay exactly what they were; this one needs a Vault next to it.
 steps:
   - name: integration
-    image: golang:1.25
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
     environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
       VAULT_VERSION: 1.20.4
       VAULT_SHA256: fc5fb5d01d192f1216b139fb5c6af17e3af742aaeffc289fd861920ec55f2c9c
       SSHSIGNHOST_TEST_VAULT_ADDR: http://127.0.0.1:8200
       # Throwaway dev-mode server on loopback; this token grants nothing.
       VAULT_TOKEN: dev-root-token
     commands:
-      # The golang image does not trust the estate's internal CA, so artifactapi
-      # is fetched over http and the archive is pinned by checksum instead.
+      # Fetched over http and pinned by checksum rather than relying on TLS trust.
       - curl -fsSL -o /tmp/vault.zip "http://artifactapi.k8s.syd1.au.unkin.net/api/v1/remote/hashicorp-releases/vault/$${VAULT_VERSION}/vault_$${VAULT_VERSION}_linux_amd64.zip"
       - echo "$${VAULT_SHA256}  /tmp/vault.zip" | sha256sum -c -
       - python3 -m zipfile -e /tmp/vault.zip /usr/local/bin/

.woodpecker/pre-commit.yaml

@@ -3,7 +3,19 @@ when:
 
 steps:
   - name: pre-commit
-    image: git.unkin.net/unkin/almalinux9-gobuilder:20260606
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
+    environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - uvx pre-commit run --all-files
     backend_options:

.woodpecker/release.yaml

@@ -12,7 +12,19 @@ clone:
 
 steps:
   - name: test
-    image: golang:1.25
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
+    environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - go test -race ./...
     backend_options:
@@ -29,11 +41,22 @@ steps:
   # One driver for both artifact types: the RPM Puppet installs via dnf and the
   # raw binaries the puppetserver-compiler initContainer fetches by URL.
   - name: goreleaser
-    image: golang:1.26
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
     environment:
       GORELEASER_VERSION: v2.17.0
       GITEA_TOKEN:
         from_secret: RELEASER_TOKEN
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - go install github.com/goreleaser/goreleaser/v2@$${GORELEASER_VERSION}
       - goreleaser release --clean

.woodpecker/test.yaml

@@ -18,7 +18,19 @@ steps:
             cpu: 2
 
   - name: test
-    image: golang:1.25
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/gobuilder:0.1.2-alma9
+    environment:
+      GOCACHE_S3_BUCKET: gocache
+      GOCACHE_S3_REGION: us-east-1
+      GOCACHE_S3_ENDPOINT_URL: https://s3.ceph.unkin.net
+      GOCACHE_S3_PATH_STYLE: "true"
+      GOCACHE_KEY_PREFIX: ci-sshsignhost
+      GOCACHE_METRICS: "true"
+      GOCACHEPROG: go-cache-plugin --cache-dir=/tmp/gocache
+      AWS_ACCESS_KEY_ID:
+        from_secret: GOCACHE_AWS_ACCESS_KEY_ID
+      AWS_SECRET_ACCESS_KEY:
+        from_secret: GOCACHE_AWS_SECRET_ACCESS_KEY
     commands:
       - go test -v -race ./...
     backend_options: