nicer entrypoint treatment

The CMS environment is not set up unless one calls shell scripts explicitly. Compare:

$ docker run -i -t --rm clelange/cmssw:5_3_32 env | sort > env1
$ docker run -i -t --rm clelange/cmssw:5_3_32 bash -c env | sort > env2
$ docker run -i -t --rm clelange/cmssw:5_3_32 bash -c 'source /home/cmsusr/.bashrc && env' | sort > env3
$ wc -l env?
    6 env1
    9 env2
  109 env3
  124 total

Note that only the latter case sets properly all the various CMS environment variables (such as CMSSW_BASE, PYTHIA8DATA and friends). However, one has to explicitly call shell startup scripts before calling a command.

This could be made nicer by introducing entry point wrapper scripts (such as explicit entrypoint.sh) that would take care of setting the environment variables as necessary before running commands. See reana-env-aliphysics for inspiration:

This gives an easier user interaction:

$ docker run -i -t --rm reanahub/reana-env-aliphysics:vAN-20180614-1 env | sort > env1
$ docker run -i -t --rm reanahub/reana-env-aliphysics:vAN-20180614-1 bash -c env | sort > env2
$  wc -l env?
   56 env1
   56 env2
  112 total

One can run commands directly without having to source anything.