add possibility for generating zstd-compressed tarballs #298

Job	Run time
eessi_container_script (help)	1m 29s
eessi_container_script (listrepos_default)	1m 9s
eessi_container_script (shell)	2m 16s
eessi_container_script (run)	3m 8s
eessi_container_script (container)	2m 9s
eessi_container_script (listrepos_custom)	1m 24s
eessi_container_script (resume)	2m 53s
eessi_container_script (unionfs)	3m 36s
	18m 4s

Provide feedback