Allow setting a default docker image to use

[dan9186]

It would be convenient if a default docker image could be set such that you could call docker dev if you have an image that you regularly use.

[brimstone]

@dan9186 is an environment variable override ok?

[dan9186]

I wouldn't really be opposed to an environment variable for it, though I was thinking something more in mind of like the git icons, tempdir, and battery module defaults.