`apt-get clean` clears out the local repository of retrieved package files.
It removes everything but the lock file from /var/cache/apt/archives/ and
/var/cache/apt/archives/partial/.

Co-authored-by: Vieno Hakkerinen <vieno@chemnitz.freifunk.net>

Updates the image to have a newer build environment, which is required
for some of the maintainer scripts.

Increases the image size from 414 to 545 MB.

Using `make container` or, if you don't have automake/gmake on your host
system, `./scripts/container.sh` will build an image for the current
branch your are on and drop you into a shell running inside a container
using that image.

From there all tooling required to work on Gluon is available.

Supports both podman (preferred) and docker.

The kernel build requires rsync.

* apt is meant for user interactive usage. apt does not guarantee a stable CLI.
* set DEBIAN_FRONTEND=noninteractive to tell apt-get that no user interaction is wanted

This will be a useful precursor for continuous integration.