Fix a 'unknown operand' warning in docker-enter

This is the kind of warning you get when adding additional operands:

  $ docker-enter $(docker ps -q -n -1) /bin/bash
  sh: /bin/bash: unknown operand
  root@60428207baf8:/#
Merge pull request #4 from jbbarth/fix-unknown-operand-warning

Fix a 'unknown operand' warning in docker-enter