ci: Use the same Docker workflow for testing as publication (#812)

* Use the same Docker build workflow in testing as is used in publication to Docker Hub
   - Amends PR #808
* Use suggested () syntax as ${{ }} is ignored in 'if' statements
   - https://twitter.com/joshmgross/status/1241088248572776451

.github/workflows/ci.yml

@@ -130,12 +130,15 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1.2.0
+    - uses: actions/checkout@v2
     - name: Build Docker image
-      run: |
-        docker build . \
-          --file docker/Dockerfile \
-          --build-arg BASE_IMAGE=python:3.7-slim \
-          --tag pyhf/pyhf:$GITHUB_SHA \
-          --compress
-        docker images
+      if: "!(startsWith(github.ref, 'refs/tags/'))"
+      uses: docker/build-push-action@v1
+      with:
+        repository: pyhf/pyhf
+        dockerfile: docker/Dockerfile
+        tag_with_sha: true
+        tag_with_ref: true
+        push: false
+    - name: List built images
+      run: docker images

.github/workflows/publish-docker.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - name: Build and Publish to Registry
-      if: "! ${{ startsWith(github.ref, 'refs/tags/') }}"
+      if: "!(startsWith(github.ref, 'refs/tags/'))"
       uses: docker/build-push-action@v1
       with:
         username: ${{ secrets.DOCKER_USERNAME }}
@@ -24,7 +24,7 @@ jobs:
         dockerfile: docker/Dockerfile
         tags: latest
     - name: Build and Publish to Registry with Release Tag
-      if: ${{ startsWith(github.ref, 'refs/tags/') }}
+      if: startsWith(github.ref, 'refs/tags/')
       uses: docker/build-push-action@v1
       with:
         username: ${{ secrets.DOCKER_USERNAME }}