Add Actions job that does a build test of Dockerfile.release

Author: tianon

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
     name: Build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2
       - name: Build
         run: |
           docker build --tag bashbrew --pull .
@@ -28,3 +28,11 @@ jobs:
           ./bashbrew list --uniq "$image"
           ./bashbrew cat "$image"
           ./bashbrew from --uniq "$image"
+  release:
+    name: Release (test)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build Dockerfile.release
+        run: |
+          docker build --pull --file Dockerfile.release .