Switch CI builds to docker buildx (#146)

## Summary
- Replace deprecated `docker build` with `docker buildx build` in the build-push-image composite action
- Remove redundant build/run comments from nettest Dockerfile

## Test plan
- [ ] Merge and tag `nettest-v1.1.0` (or similar) to trigger the build-container workflow
- [ ] Verify the build succeeds without the deprecation warning

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/146

.forgejo/actions/build-push-image/action.yaml

@@ -30,7 +30,7 @@ runs:
       shell: bash
       run: |
         echo "Building ${{ inputs.image_name }}:${{ inputs.version }}"
-        docker build \
+        docker buildx build \
           --tag ${{ inputs.image_name }}:${{ inputs.version }} \
           --file ${{ inputs.context }}/${{ inputs.dockerfile }} \
           ${{ inputs.context }}