Env class yet to be implemented
the object
Other instruction classes: Add-class,
Arg-class, Cmd-class,
Comment-class, Copy-class,
Entrypoint-class,
Expose-class, From-class,
Healthcheck-class,
Instruction-class,
Label-class,
Maintainer-class,
Onbuild-class, Run-class,
Run_shell-class, Shell-class,
Stopsignal-class,
Volume-class, Workdir-class
#no example yet