add support to `eessi_container.sh` for using `unionfs` overlay tool #60

Job	Run time
eessi_container_script (listrepos_custom)	1m 48s
eessi_container_script (run)	1m 16s
eessi_container_script (shell)	1m 15s
eessi_container_script (listrepos_default)	29s
eessi_container_script (unionfs)	1m 28s
eessi_container_script (help)	23s
eessi_container_script (resume)	1m 14s
eessi_container_script (container)	1m 14s
	9m 7s

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