Previously, config_stripper would store the full
TAR file and gzipped TAR file in memory to compute
the SHA256 hash.

After this change, the stripped TAR file is stored
in a temporary location on disk and then renamed
into its final location.  Furthermore, the SHA256
hash of the gzipped TAR file is computed
incrementally as the gzipping progresses.

Previously, install_pkgs would mount files into
the target image directly (with '-v src:dst').
This unfortunately doesn't work with
docker-in-docker (DIND) setups where the mount
is done from the perspective of the host which
does not share a filesystem with the docker client.

To work with such setups, install_pkgs now creates
a named volume and copies files into the named
volume.  The named volume containing the install
script and TAR file is then mounted into the
target container.

Not all images have a command, so we need to
specify one.