Better var name (make it more private)

Docker-DCO-1.1-Signed-off-by: Mangled Deutz <olivier@webitup.fr> (github: dmp42)
Kill the magic

Docker-DCO-1.1-Signed-off-by: Mangled Deutz <olivier@webitup.fr> (github: dmp42)
Replace command exec by env var

Docker-DCO-1.1-Signed-off-by: Mangled Deutz <olivier@webitup.fr> (github: dmp42)