GitHub: only build docker images for 64bit ARM

To disincentivize users of 32bit machines to use lnd, we removed the
ARMv7 (32bit) target for docker images there. It therefore doesn't make
sense to provide LiT images for 32bit machines either.

Author: guggero

.github/workflows/docker.yml

@@ -38,7 +38,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           push: true
-          platforms: linux/amd64,linux/arm/v7,linux/arm64
+          platforms: linux/amd64,linux/arm64
           tags: "${{ env.DOCKER_REPO }}/${{ env.DOCKER_IMAGE }}:${{ env.RELEASE_VERSION }}"
           build-args: checkout=${{ env.RELEASE_VERSION }}