Add helper to run a containerized build environment (#2292)

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.