Update docs to note that btrfs example requires a baremetal install

[iluvatyr]

Is your feature request related to a problem? Please describe.
In the docker image, I can not yet use the btrfs utilities to create btrfs snapshots, yet it is shown as a possible hook here

Describe the solution you'd like
Add btrfs-progs to the image or add functionality that lets users add specific packages via environment variables (e.g.
ADDITIONAL_PACKAGES=btrfs-progs,...,zip

[garethgeorge]

BTRFS commands are a bit too niche to include in the image by default, I think this is a good candidate for a custom image or I'd more strongly recommend a bare metal install of backrest if running filesystem commands

As a side note: are btrfs userspace commands usable from docker to manage filesystems on the host? Assuming the container needs to be privileged and possibly needs sockets mounted?

[iluvatyr]

I actually have no idea if it is possible to manage btrfs filesystems from within a docker image. That would have to be checked, I just (naively) assumed it is possible.

Then just as a side node, I would maybe add to the documentation there on the example that it is not possible if deployed as a container? Not absolutely necessary, but maybe someone doesn't know