docker development functions

[benmarwick]

@sckott just told me about this package via the thread on r-lib/devtools#710 Do you think it would make sense to have those four functions mentioned in that thread (or ones like them) in this package?

[wch]

I think that the build and push functions would fit in here, but add_dockerfile and add_circle don't quite seem right for harbor.

[benmarwick]

Thanks, so do you think add_dockerfile and add_circle might be best in devtools, or somewhere else?

[wch]

Hm, hard to say. I could see it either way. I'd defer to Hadley's opinion on that.

[karthik]

Doesn't make sense for devtools but Hadley should decide.