github: Add action to test build docker image

.github/workflows/docker-build.yml

@@ -0,0 +1,15 @@
+name: Docker Image
+on:
+  push:
+    branches: [ "master", "develop" ]
+  pull_request:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: docker/build-push-action@v1
+        with:
+          repository: inbucket/inbucket
+          push: false
+          tag_with_ref: true