Add --container-subvolume-relpath flag for init

[muir]

This is to allow the same source to be backed up to multiple destinations

[masc3d]

is it really necessary to be fully customizable path?

I would much prefer to support multi-destination via names, path convention and a more concise / intuitive option than --container-subvolume-relpath eg.

ref. #9

[masc3d]

how about

• -n --name backup name
• local subvol .sxbackup-$name

[muir]

how about

• -n --name backup name
• local subvol .sxbackup-$name

Would that also change the naming for the other destination backups?
Currently they're things like sx-20210221-065715-utc but would that become
$name-yyyymmdd-hhmmss-utc instead?

Would you like me to do that work? My python is quite rusty: this PR was the first I had touched it in 5 years.

[muir]

I presume the destination must remain dest/.btrfs-sxbackup because otherwise you would have to specify the name with every command.

[masc3d]

Would that also change the naming for the other destination backups?
$name-yyyymmdd-hhmmss-utc instead?

each job should still have dedicated destination, so I don't believe it's necessary.
why would it be beneficial to include name in snapshot volumes?

Would you like me to do that work? My python is quite rusty: this PR was the first I had touched it in 5 years.

same here xD I don't need this feature, so it's up to you "how far" you want to go.
just want to make sure command line interface remains clean, as cleaning up later could be breaking change.

enhancing init as a first step will work just fine, no? this would be rather minimal change to your PR.