Sometimes, during development or when fixing a deployed app, you want to get inside the container environment to poke around. Just to work out why the hell something did or didn't happen.

Right now it is difficult. Docker doesn't support it directly. There is a lot of demand for it:

https://github.com/docker/docker/issues/1228

You have to use nsenter, which is OK, but requires additional installation:

https://github.com/jpetazzo/nsenter

It looks as though an-almost-but-not-quite-the-same thing will land in Docker at some point:

https://github.com/docker/docker/pull/7409

And this shell is another attempt to make a very useful feature available.