Split docker into separate CI task

.github/workflows/build.yml

@@ -17,5 +17,3 @@ jobs:
       run: pip install -r dev-requirements.txt
     - name: Run tests
       run: bash ./scripts/test.sh
-    - name: Build docker image
-      run: docker build .

.github/workflows/docker.yml

@@ -0,0 +1,13 @@
+name: Build Docker Image
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Build docker image
+      run: docker build .