Add Sections on Containers and CI

[jhwgh1968]

• [x] Everything I'm contributing is my own work, used with permission (and attributed as necessary), or is very clearly fair use
• [x] I agree to put my contribution into the Public Domain (or CC-0, if that doesn't apply where you are)

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How about explaining the basics of containers, and then making CI for your open source project the example? Two birds, one stone!

The container section in Servers in is focused on the concepts, but hopefully is not too brief and has good choices of further reading. You don't want to re-write the docker manual, after all.

Unless @docquantum had something else in mind, also closes #2

[VegaDeftwing]

I love how succinct you made this. Thank you for the fantastic contribution!