Figure out how to use our Docker images locally

[Minoru]

We have a bunch of Dockerfiles which are used by our CI and can also be utilized by users who want to build Newsboat from source. These Dockerfiles have instructions on how to run the images locally:

newsboat/docker/ubuntu_22.04-build-tools.dockerfile

Lines 38 to 43 in 542800c

	# docker run \
	# --rm \
	# --mount type=bind,source=$(pwd),target=/home/builder/src \
	# --user $(id -u):$(id -g) \
	# newsboat-build-tools \
	# make

There are two problems with this:

1. it requires users inside and outside Docker to match (have same GID and UID), which might not be the case if e.g. a machine has multiple user accounts;
2. it means our Dockerfiles have some hard-coded UID and GID (1000 and 1000 currently), so people know what UID/GID they have to provide.

This worked well enough so far, probably because people who tried Docker locally only had one user account on their machine (with UID/GID 1000/1000). However, Ubuntu 23.04 image for Docker comes with "ubuntu" user, so we'd have to adjust the recipes for it.

This is an opportunity to re-think how we do local builds with Docker. The goals are:

1. Dockerfiles can be used in CI
2. Dockerfiles can be used to produce local Docker images, from which one can start a container with Newsboat sources mapped into it, and build from that source
3. the instructions to perform item 2 should be uniform for all our Dockerfiles. It's okay if Dockerfiles themselves are quite different

We could probably achieve that if we deleted the default "ubuntu" user in Ubuntu 23.04, but I'm not sure if that won't break something. We could also look for alternatives; perhaps the existing docker run --user ... --mount ... approach is wrong.