Add conditions for pushing images from GithubCI to DockerHub

This commit restricts the condition for GithubCI to push the docker 
image to DockerHub, so the pull requests will perform the GithubCI 
build runs but it won't pollute the DockerHub images by pushing the 
built images up there.

Author: Chun-Hung Tseng

.github/workflows/docker.yml

@@ -20,6 +20,7 @@ jobs:
         uses: docker/setup-buildx-action@v3
       - name: Login to Docker Hub
         uses: docker/login-action@v3
+        if: github.event_name == 'push'
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_ACCESS_TOKEN }}
@@ -28,5 +29,5 @@ jobs:
         with:
           context: .
           platforms: linux/amd64,linux/arm64
-          push: true
-          tags: sysprog21/chisel-bootcamp:latest
+          push: ${{ github.event_name == 'push' }}
+          tags: sysprog21/chisel-bootcamp:latest