Draft: Build server in container to reduce build time

Objective

VM-in-VM managed by vagrant => podman-in-VM managed by vagrant

Requirements

• No breaking changes. All changes are enabled by flags.
• Use registry.gitlab.com/fdroid/docker-executable-fdroidserver:master or registry.gitlab.com/fdroid/ci-images-client.

Tasks

• build_server in fdroidserver/build.py
• force_halt_build in fdroidserver/build.py
• makebuildserver
• fdroid-bootstrap-buildserver

Alternatives

• podman-in-podman without vagrant or any other dependencies. On host you only need to install podman and a image.