Add docker / gitpod integrations

[alexjbest]

Mathlib has .docker and .devcontainer folders that (for one thing) make it easy to quickly load in to a fresh install of lean + mathlib (eg with gitpod.io). I think adding these to Std would be helpful

[kim-em]

PR welcome! :-)

Note that we're hoping to distribute Std directly with Lean itself (i.e. no require std required for projects that use it), but I think these integrations would remain just as useful even if we can pull that off.

[alexjbest]

Ok, I'll make a PR when I get a chance!

[kim-em]

Partially resolved by #732, but perhaps someone should still copy over Mathlib's .devcontainer/*, deleting the line about lake exe cache get.