add support to `eessi_container.sh` for using `unionfs` overlay tool #54

Job	Run time
eessi_container_script (run)	1m 11s
eessi_container_script (shell)	1m 31s
eessi_container_script (listrepos_custom)	1m 35s
eessi_container_script (listrepos_default)	1m 5s
eessi_container_script (container)	1m 11s
eessi_container_script (resume)	2m 54s
eessi_container_script (unionfs)	2m 23s
eessi_container_script (help)	1m 46s
	13m 36s

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