Only run Docker CI stages when necessary.

Instead of building the image all the time, and only pushing if
necessary.

.github/workflows/dicebot-ci.yml

@@ -30,7 +30,7 @@ jobs:
     needs: build_and_test
 
     runs-on: ubuntu-latest
-    if: github.event_name == 'push'
+    if: github.event_name == 'push' && github.ref == 'refs/heads/master'
 
     steps:
       - name: Checkout code
@@ -43,13 +43,12 @@ jobs:
           password: ${{ secrets.DOCKER_PASSWORD }}
           repository: projectmoon/chronicle-dicebot
           tags: latest
-          push: ${{ github.ref == 'refs/heads/master' }}
 
   docker_push_tag:
     needs: build_and_test
 
     runs-on: ubuntu-latest
-    if: github.event_name == 'push'
+    if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags/')
 
     steps:
       - name: Checkout code
@@ -61,6 +60,4 @@ jobs:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_PASSWORD }}
           repository: projectmoon/chronicle-dicebot
-          tags: latest
           tag_with_ref: true
-          push: ${{ startsWith(github.ref, 'refs/tags/') }}