Add binstub to easily cleanout old docker files

[wopian]

Clean up several GBs of wasted space from old builds.

[cybrox]

I'd probably call it clean-docker instead of clean, for clarity.

Also, the last line will most likely fail on OS X. I'd put an ìf -d` before it, thought it probably won't do much when it tries to search in a nonexistent directoy. :bomb:

[cybrox]

LGTM