Add git submodule plato to CI's conditional paths

Author: randomnoise

.github/workflows/docker.yaml

@@ -1,18 +1,18 @@
+# Experiments:
+# [x] Can I read env.IMAGE_NAME?
+# [x] Can I print git event name?
+# [x] Can I recognize pull requests?
+# [] Can I use if conditions for steps?
 name: Docker CI
 
 on:
   push:
-    branches: main
-    paths:
-      - '*.Dockerfile'
-      - '*.Dockerfile.dockerignore'
-      - '.github/workflows/*.yaml'
-  pull_request:
-    branches: main
+    branches: try-ci-conditionals
     paths:
       - '*.Dockerfile'
       - '*.Dockerfile.dockerignore'
       - '.github/workflows/*.yaml'
+      - plato
 
 env:
   REGISTRY: ghcr.io
@@ -42,34 +42,20 @@ jobs:
     # https://github.com/docker/metadata-action
     - name: Extract Docker metadata
       id: meta
-      uses: docker/metadata-action@v5
-      with:
-        images: ${{ env.IMAGE_NAME }}
-        flavor: latest=true
+      if: ${{ github.event_name != 'pull_request' }}
+      run: echo 'Extract Docker metadata'
 
     - name: Log into registry ${{ env.REGISTRY }}
-      uses: docker/login-action@v3
-      with:
-        registry: ${{ env.REGISTRY }}
-        username: ${{ github.actor }}
-        password: ${{ secrets.GITHUB_TOKEN }}
+      if: ${{ github.event_name != 'pull_request' }}
+      run: echo 'Log into registry'
 
     # Build and push Docker image with Buildx (don't push on PR)
     # https://github.com/docker/build-push-action
     - name: Build and push Docker image
       id: build-and-push
-      uses: docker/build-push-action@v6
-      with:
-        file: ${{ matrix.purpose }}.Dockerfile
-        context: ./plato
-        push: ${{ github.event_name != 'pull_request' }}
-        tags: ${{ steps.meta.outputs.tags }}
-        labels: ${{ steps.meta.outputs.labels }}
+      run: echo 'Build and push Docker image'
 
     - name: Check image layers
+      if: ${{ github.event_name != 'pull_request' }}
       id: check-layers
-      run: |
-        set -eux -o pipefail
-        docker pull ${{ env.IMAGE_NAME }}
-        docker images
-        docker image history ${{ env.IMAGE_NAME }}
+      run: echo 'Check image layers'