add support to `eessi_container.sh` for using `unionfs` overlay tool #45

Job	Run time
eessi_container_script (listrepos_default)	1m 52s
eessi_container_script (resume)	3m 39s
eessi_container_script (help)	1m 46s
eessi_container_script (run)	2m 38s
eessi_container_script (shell)	2m 4s
eessi_container_script (listrepos_custom)	1m 58s
eessi_container_script (container)	2m 30s
eessi_container_script (unionfs)	1m 56s
	18m 23s

Provide feedback