.github/workflows: drop linux/arm/v7 from docker

I believe chainguard no longer supports arm/v7.  This was added at a
user's request I think that was running imageproxy on a raspberry pi or
something.  I might switch to a different base image that does have
support, though it's annoying to have to do so.  In the meantime, users
can always built the image themselves for other platforms.

Author: willnorris

.github/workflows/docker.yml

@@ -54,7 +54,7 @@ jobs:
           push: ${{ github.event_name == 'push' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          platforms: linux/amd64,linux/arm64,linux/arm/v7
+          platforms: linux/amd64,linux/arm64
 
       # Sign the Docker image
       - name: Install cosign