add support to `eessi_container.sh` for using `unionfs` overlay tool #61

Job	Run time
eessi_container_script (listrepos_custom)	52s
eessi_container_script (help)	1m 25s
eessi_container_script (container)	1m 45s
eessi_container_script (run)	1m 49s
eessi_container_script (resume)	1m 26s
eessi_container_script (shell)	1m 41s
eessi_container_script (unionfs)	1m 48s
eessi_container_script (listrepos_default)	1m 3s
	11m 49s

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