Execute docker commands from within the container

[fritz-k]

Hi,

first of all thanks for maintaining this docker container, it looks like an - almost - perfect fit for our backups.

Would you mind adding the docker-cli package to the dockerfile - either directly or via a seperate tag? My goal is to bind-mount the docker socket to this container and execute docker commands in the pre- and post-hooks - e.g. shut down containers during backup.

Cheers,
Simon