Run docker build on all PRs, but only push image on merge

Signed-off-by: Matthew Whitehead <matthew1001@gmail.com>

.github/workflows/docker_main.yml

@@ -2,8 +2,9 @@ name: Docker Main Build
 
 on:
   push:
-    branches:
-      - main
+    branches: [main]
+  pull_request:
+    branches: [main]
 
 jobs:
   docker:
@@ -32,14 +33,17 @@ jobs:
             --tag ghcr.io/hyperledger/firefly-tokens-erc1155:${{ steps.build_tag_generator.outputs.BUILD_TAG }} .
 
       - name: Tag release
+        if: github.event.pull_request.merged == true
         run: docker tag ghcr.io/hyperledger/firefly-tokens-erc1155:${{ steps.build_tag_generator.outputs.BUILD_TAG }} ghcr.io/hyperledger/firefly-tokens-erc1155:head
 
       - name: Push docker image
+        if: github.event.pull_request.merged == true
         run: |
           echo ${{ secrets.GITHUB_TOKEN }} | docker login ghcr.io -u $GITHUB_ACTOR --password-stdin
           docker push ghcr.io/hyperledger/firefly-tokens-erc1155:${{ steps.build_tag_generator.outputs.BUILD_TAG }}
 
       - name: Push head tag
+        if: github.event.pull_request.merged == true
         run: |
           echo ${{ secrets.GITHUB_TOKEN }} | docker login ghcr.io -u $GITHUB_ACTOR --password-stdin
           docker push ghcr.io/hyperledger/firefly-tokens-erc1155:head