Bonjour,
While upgrading the forum I also removed dangling images to recover ~5GB as follows:
ubuntu@forum:/var/discourse$ sudo docker image prune
WARNING! This will remove all dangling images.
Are you sure you want to continue? [y/N] y
Deleted Images:
deleted: sha256:80c8404c82626ce1f119413203182c086647f52c65955c4299549496541b0387
deleted: sha256:cc1e4d67602dd2326426b27a77c6dcc0f61a9f10870f2d790bef042a7233bd0a
deleted: sha256:619b9b4e956e6f32924be499c53b9fb710e5b65c95bfd1bf08291b5b59d29c78
deleted: sha256:7bf43c8bb85dfe199095a14eab73a68727ff7a9de4141e9498580eb00a88771f
deleted: sha256:28102bd837b22a7799d6eb91dbf778dd74146a8b3775596335ba790536490d13
deleted: sha256:dfefa32acc3e75dab1fad19ee92582abaa1f535fa7928a62eaab49b035a6f0f1
deleted: sha256:a4a87fa624c79d94f040906d0406b640ebbf630dbfd48990165d2cc84404f66f
deleted: sha256:a97a927bcec5649ac2a095f673af5fac1291a90f95b4399f6142b7d081020c5a
deleted: sha256:d2f7fcbad8aa758d92f60ce98e6635e5cd98978b5342e936ecf1054c2a37baeb
deleted: sha256:fb87d2977cbb62255ddc58fd210b437cda81f94987865cdfe6e02645957228e0
Total reclaimed space: 4.917GB
Cheers