add support to `eessi_container.sh` for using `unionfs` overlay tool #47

Job	Run time
eessi_container_script (help)	1m 26s
eessi_container_script (container)	2m 28s
eessi_container_script (listrepos_custom)	1m 17s
eessi_container_script (listrepos_default)	1m 24s
eessi_container_script (resume)	3m 26s
eessi_container_script (run)	2m 58s
eessi_container_script (shell)	2m 19s
eessi_container_script (unionfs)	2m 47s
	18m 5s

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