ci: disable publishing PR images to docker hub

Docker hub is rate limiting our API access, as a result tests are
failing for no apparent reason. This change will decrease the API calls
by at least 80%

We have applied for an Open Source account with greater API limits, I
will come back to this and update as necessary when and if they grant us
access to their program.

.github/workflows/dev_container.yml

@@ -27,6 +27,7 @@ jobs:
 
       - name: Login to Docker Hub
         uses: docker/login-action@v3
+        if: github.event_name != 'pull_request'
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_TOKEN }}
@@ -44,7 +45,7 @@ jobs:
         with:
           images: |
             ghcr.io/PX4/px4-dev
-            px4io/px4-dev
+            ${{ (github.event_name != 'pull_request') && 'px4io/px4-dev' || '' }}
           tags: |
             type=schedule
             type=semver,pattern={{version}}