Document how to use dep with Docker

[carolynvs]

Now that dep ensure -vendor-only works in directory without go code (i.e. you copied just Gopkg.toml/lock into a container), we should explain how to use that in the FAQ: "How do I use dep with Docker?".

#954 has a nice example of how it works.

Once we tackle #957, the setup will be even easier.

[stephenafamo]

Looking for how to help. Can I try this? @carolynvs

[carolynvs]

@stephenafamo Go for it! 💥