ci: build mkdocs image periodically

.woodpecker/doc-image-builder.yml

@@ -0,0 +1,50 @@
+labels:
+  platform: linux/amd64
+
+when:
+  - event: push
+    branch:
+      - main
+    path: &paths
+      include:
+        - .woodpecker/doc-image-builder.yml
+        - "docs/Dockerfile"
+        - "crates/toml2jsonc/**"
+  - event: pull_request
+    path: *paths
+  - event: manual
+  - event: cron
+    cron: build-builder
+
+variables:
+  - &docker_credentials
+      username:
+        from_secret: gh_package_publish_user
+      password:
+        from_secret: gh_package_publish_token
+
+steps:
+  build-builder-no-publish:
+    image: plugins/docker
+    when:
+      - evaluate: 'CI_PIPELINE_EVENT == "pull_request" || (CI_PIPELINE_EVENT == "push" && CI_COMMIT_BRANCH != "main")'
+    settings:
+      registry: ghcr.io
+      repo: "ghcr.io/kumocorp/mkdocs-material"
+      dry_run: true
+      tags:
+        - latest
+      dockerfile: docs/Dockerfile
+
+  publish-builder:
+    image: plugins/docker
+    when:
+      - evaluate: '(CI_PIPELINE_EVENT != "pull_request") && CI_COMMIT_BRANCH == "main"'
+    settings:
+      <<: *docker_credentials
+      registry: ghcr.io
+      repo: "ghcr.io/kumocorp/mkdocs-material"
+      tags:
+        - latest
+      dockerfile: docs/Dockerfile
+