Restore docker buildx build (#149)

## Summary
- Switch build action back to `docker buildx build` now that runner v2.5.1 (with `docker-buildx-plugin`) is deployed

## Test plan
- [ ] Merge and tag `nettest-v0.12.0` to verify buildx works end-to-end

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/149

.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 }}