Add support for zstd compression

[michalmiddleton]

Hi,
I took a stab at adding zstd support in a way that keeps gzip as default to maintain backward compatibility:

• added BACKUP_COMPRESSION parameter (gzip/zstd)
• updated BACKUP_FILENAME to accept '' for extension that will dynamically change to tar.gz or tar.zst depending on compression
• added a build test cli-zstd

Please let me know if you require any additional changes.

Real world test:

backups  | time=2023-08-17T19:29:00.451-05:00 level=INFO msg="Stopping 3 container(s) labeled `docker-volume-backup.stop-during-backup=true` out of 16 running container(s)."
backups  | time=2023-08-17T19:30:02.818-05:00 level=INFO msg="Created backup of `/backup` at `/tmp/backup-2023-08-17T19-29-00.tar.zst`."
backups  | time=2023-08-17T19:30:04.637-05:00 level=INFO msg="Restarted 3 container(s) and the matching service(s)."
backups  | time=2023-08-17T19:31:48.106-05:00 level=INFO msg="Removed tar file `/tmp/backup-2023-08-17T19-29-00.tar.zst`."
backups  | time=2023-08-17T19:31:48.106-05:00 level=INFO msg="Finished running backup tasks."

Extract to validate consistency:

root@archer:/data# unzstd backup-2023-08-17T19-29-00.tar.zst
backup-2023-08-17T19-29-00.tar.zst: 17936120320 bytes

root@archer:/data# tar tf backup-2023-08-17T19-29-00.tar > /dev/null
tar: Removing leading `/' from member names
root@archer:/data#

(The uncompressed tar was about 17GB in size and zstd compressed archive was about 7GB!)

[m90]

Thanks for working on this feature 🎩 I left a few comments inline, nothing major. Let me know what you think about my suggestions.

[m90]

Thanks for getting this done so fast again. I left another round of nit picky comments, the only one that is really interesting is the one about maybe having a custom type in the config that limits the number of possible compression algorithms. Let me know what you think.

[m90]

It would also be possible to anonymously define the data being passed here

Suggested change

	if tErr := tmplFileName.Execute(&bf, extMap); tErr != nil {
	if tErr := tmplFileName.Execute(&bf, map[string]string{
	"Extension": "tar."+s.c.BackupCompression
	}); tErr != nil {

[michalmiddleton]

@m90 Thanks for being patient with me :). I pushed amended commit with the changes.

[m90]

One more tiny remark, everything else looks fabulous and ready for the next release now.