Mask some paths of the outer container in the OCI config of the inner
container. This is done to avoid leaking any sensitive information from
Podman / Docker / gVisor, since we reuse the same rootfs
Refs #1048
Remove the need to copy the Dangerzone container image (used by the
inner container) within a wrapper gVisor image (used by the outer
container). Instead, use the root of the container filesystem for both
containers. We can do this safely because we don't mount any secrets to
the container, and because gVisor offers a read-only view of the
underlying filesystem
Fixes#1048
Download and copy the following artifacts that will be used for building
a Debian-based Dangerzone container image in the subsequent commits:
* The APT key for the gVisor repo [1]
* A helper script for building reproducible Debian images [2]
[1] https://gvisor.dev/archive.key
[2] d15cf12b26/repro-sources-list.sh
Move container-only build context (currently just the entrypoint script)
from `dangerzone/gvisor_wrapper` to `dangerzone/container_helpers`.
Update the rest of the scripts to use this location as well.
