Investigate if a variant build can be built inside a container

A customer attempted to build bottlerocket variants inside of a docker container with the docker daemon socket shared between the host and the container. The build failed when buildsys orchestrated docker build to mount the sources into the build context because of the directory going from container volume -> host -> build-context. (https://github.com/bottlerocket-os/bottlerocket/issues/4100)

We should investigate whether it would be possible for this flow to work. Roughly we could allow the user to specify a host path that is shared via a mounted volume in both the build container and then in the build context.

Start by tracing how buildsys orchestrates docker build and mounts sources into the build context. Review the related Bottlerocket issue 4100 and assess whether a host path can be shared through mounted volumes in both the build container and build context. Done means documenting feasibility, constraints, and the required implementation scope.

Written by the indexing model from the issue text.