Check free disk space before starting Docker (#2092)

[agners]

It seems that Docker can fail to start if there is no space left on the
device. Try to free up some space in that case by asking journald to
limit its size to 256MiB.

This should work for any storage larger than ~2.5GiB (as the journals
maximum size is 10% of the disk size). It still should leave enough
logs to diagnose problems if necessary.

Note: We could also limit the size of the journal in first place, but
that isn't sustainable: Once that space is used up, we run into the
same problem again.

By only asking journalctl to free up if necessary, we kinda (miss)use
the journal as way to "reserve" some space which we can free up at boot
if necessary.

[agners]

This is a bit crude, but it seems to work in my testing. The regular root reserve unfortunately doesn't work as most services run in containers as root. Better would be to use actual disk quota, and enforcement. But disk quotas are a pain to manage, and there is overhead associated to it.

An alternative to the current solution would be to store a "reserve" file, which we can delete at startup. But that is rather crude too, and leads to unnecessary writes (as the file needs to really be written out, a sparse file would not do the job here).