Bug: ghcr images are lacking arm64

# Describe the bug

The images provided in ghcr.io/foxcpp/maddy seem to lack arm64.

# Steps to reproduce

1. Try to pull Maddy on arm64 host from ghcr.io.
2. See it fail due to manifest not including anything other than amd64

# Log files

```
docker pull ghcr.io/foxcpp/maddy:0.8.1
0.8.1: Pulling from foxcpp/maddy
no matching manifest for linux/arm64/v8 in the manifest list entries
```
