Dockerization docs update

[blyedev]

I spent the last few weeks exploring what modes of dockerization are applicable to renv and I thought it might be worth documenting them which is why I prepared the #2237 PR. I have reworked the vignette completely removing sections like the runtime restore in favor of the host level cache build strategy which achieves the same effect without violating the 5th principle of a 12 factor app.

Because I spent a lot of time working on docker images I might have left out some things considering them self explanatory but I've tried my best to explain those. Please let me know if I missed any.

[kevinushey]

Implemented in #2237.