Use volume mounts to avoid including the Portage tree in your images:

    $ docker run -d --name portage wking/portage

This exports a [VOLUME][] which you can [mount][volumes-from] from
another container:

    $ docker run --volumes-from portage -i -t wking/gentoo /bin/bash
    d1a49abc4b3c / # ls /usr/portage/
    app-accessibility  dev-python        mail-mta         sci-mathematics
    …

Changes (e.g. distfiles downloads) are preserved between mounts by the
continuously-running `portage` container.  Let's install something in
the first container:

    d1a49abc4b3c / # emerge -av netcat
    …
    These are the packages that would be fetched, in order:

    Calculating dependencies... done!
    [ebuild  N     ] dev-libs/libmix-2.05-r6  USE="-static-libs" 78 kB
    [ebuild  N     ] net-analyzer/netcat-110-r9  USE="crypt ipv6 -static" 108 kB

    Total: 2 packages (2 new), Size of downloads: 186 kB

Now kill that container and spin up another one:

    $ docker run --volumes-from portage -i -t wking/gentoo /bin/bash
    187adaf8babd / # emerge -pv netcat
    …
    These are the packages that would be merged, in order:

    Calculating dependencies... done!
    [ebuild  N     ] dev-libs/libmix-2.05-r6  USE="-static-libs" 0 kB
    [ebuild  N     ] net-analyzer/netcat-110-r9  USE="crypt ipv6 -static" 0 kB

    Total: 2 packages (2 new), Size of downloads: 0 kB
    …

The local Portage cache, read news items, etc. stored outside of
`/usr/portage` (e.g. in `/var/cache/edb`, `/var/lib/gentoo/news`, …)
will still be local to your client containers, so you'll get
promptings for reading the news on both `d1a49abc4b3c` and
`187adaf8babd`.

You can use container volumes even if their container is not running.
For example:

    $ docker run --name portage wking/portage true

However, it may be useful to leave the container running so you don't
remove it up by accident.

[VOLUME]: http://docs.docker.io/en/latest/use/builder/#volume
[volumes-from]: http://docs.docker.io/en/latest/use/working_with_volumes/#mount-volumes-from-an-existing-container