L2SS-188: Renamed statistics_device -> Statistics, as it's a name, and it's a device per definition.

Any images missing from the artefact.skatelescope.org registry are now
configured to be downloaded from git.astron.nl:5000

This patch ensures all images available from artefact.skatelescope.org
are pinned to a specific version. This is necessary as :latest is not
available on this registry.

L2SS-251: Refer to the DOCKER_REGISTRY environment variables in .env for source images, instead of hard-coding them in the Dockerfiles

L2SS-217: Run JIVE container on host network, instead of the custom NETWORK_MODE, to allow it to connect to DISPLAY.

L2SS-194: Build PCC & SDP devices explicitly, since docker compose doesn't understand running multiple services from a single local base image.

Use proc fs to access sysctl values, to be able to query whether the variables we want to write are exposed and writable at all.