add support to `eessi_container.sh` for using `unionfs` overlay tool #75

Job	Run time
eessi_container_script (help)	1m 4s
eessi_container_script (container)	2m 57s
eessi_container_script (unionfs)	2m 41s
eessi_container_script (resume)	1m 57s
eessi_container_script (listrepos_default)	2m 6s
eessi_container_script (listrepos_custom)	1m 18s
eessi_container_script (run)	3m 15s
eessi_container_script (shell)	1m 45s
	17m 3s

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