This change is the first step of integrating appendable string arrays
into containers.conf and starts with enabling the `Env`, `Mounts`, and
`Volumes` fields in the `[Containers]` table.
Both, Buildah and Podman, read (and sometimes write) the fields of the
`Config` struct at various places, so I decided to migrate the fields
step-by-step. The ones in this change are most critical ones for
customers. Once all string slices/arrays are migrated, the docs of
containers.conf will be updated. The current changes are entirely
transparent to users.
Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>
