the documentations says that you can drop "a file" into the
`docker.service.d` directory, but does not note that the file must end
with `.conf` in order to be recognized by systemd. This can lead to
some [confusion][] if readers are not previously familiar with
systemd.
[confusion]: https://botbot.me/freenode/docker/2016-05-06/?msg=65605541&page=11
Signed-off-by: Lars Kellogg-Stedman <lars@redhat.com>
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