By default, docker build downloads the DCP-o-matic command-line package from
dcpomatic.com.
If that download does not work for you (no internet access on the NAS, a
version that is not the default, another architecture...), download the
Ubuntu 24.04 CLI .deb yourself from
https://dcpomatic.com/download, put it in this folder and build again:
it will be installed instead.
.deb files in this folder are ignored by git.