Add linting using unstable & oldstable containers

Use the same matrix of containers for linting in the
`Validate Codebase` GHAW that we're already using
for testing and building jobs.

This applies the minimum linting requirements in addition
to testing "unstable" linting options that may become the
new baseline in the future.

refs GH-206

.github/workflows/lint-and-build-code.yml

@@ -19,9 +19,23 @@ jobs:
   lint_code:
     name: Lint codebase
     runs-on: ubuntu-latest
+
+    # Don't flag the whole workflow as failed if "experimental" matrix jobs
+    # fail. This allows the unstable image linting tasks to fail without
+    # marking the oldstable and stable image linting jobs as failed.
+    continue-on-error: ${{ matrix.experimental }}
     timeout-minutes: 10
+    strategy:
+      # Don't stop all workflow jobs if the unstable image linting tasks fail.
+      fail-fast: false
+      matrix:
+        container-image: ["go-ci-oldstable", "go-ci-stable", "go-ci-unstable"]
+        include:
+          - container-image: "go-ci-unstable"
+            experimental: true
+
     container:
-      image: index.docker.io/atc0005/go-ci:go-ci-lint-only
+      image: "index.docker.io/atc0005/go-ci:${{ matrix.container-image}}"
 
     steps:
       - name: Check out code