Add armv7 support in docker images

[Chuckame]

Currently, only amd64 image is available on docker hub.

Can you add it for armv7 ?

As indicated here, the only change would be inside your github actions:

      [...]
      - name: Build and push
        uses: docker/build-push-action@v5
        with:
          [...]
          platforms: linux/amd64,linux/armv7

Same as https://github.com/rapiz1/rathole/pull/285. All the platforms that rathole supports could have its own docker image for a simpler usability.

[realchandan]

Much needed, also arm64