There were some visual aspects to Tcl that always bothered me despite smart people telling me that it has fairly well thought out structure.
I've gravitated towards Lua (because I was easily able to integrate C libs there), but been pushed into the Python camp simply because there's lots of stuff I don't have to write to get stuff done.
I've not used Lua but I would be surprised if it's easier to link it with C libs than Tcl! Because it is what Tcl was designed for, you have some functionality in C and want to add scriptability to it. It just happens to be an excellent, highly productive, general purpose language as well!
That's what I was told, but I guess I was suffering from "learn-the-new-thing-itis" so I invested in Lua... I've been meaning to give Tcl a fair shake though so this Docker API might be a good motivator.
and also Learn Tcl in Y minutes: http://learnxinyminutes.com/docs/tcl/
(this may be the fastest way of getting basic understanding of Tcl)