ci: validate observer Docker builds

.github/workflows/observer-build.yaml

@@ -0,0 +1,44 @@
+name: Observer Docker build
+
+on:
+  pull_request:
+    paths-ignore:
+      - "**/*.md"
+      - "**/*.txt"
+  push:
+    branches:
+      - main
+    paths-ignore:
+      - "**/*.md"
+      - "**/*.txt"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: observer-build-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  observer-build:
+    name: Build observer image
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    steps:
+      - name: Check out code
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Set up Docker buildx
+        uses: docker/setup-buildx-action@37fe631027851001ddb9b187196cc803df7f5f0e # v4.3.0
+
+      - name: Build observer image
+        uses: docker/build-push-action@53b7df96c91f9c12dcc8a07bcb9ccacbed38856a # v7.3.0
+        with:
+          context: .
+          file: tools/observer/Dockerfile
+          platforms: linux/amd64
+          push: false
+          cache-from: type=gha,scope=observer-build
+          cache-to: type=gha,mode=max,scope=observer-build