refactor: refactor envbuilder to use coder/serpent as CLI engine

[BrunoQuaresma]

Related to #130

[mtojek]

I think you can review clidocgen in coder/coder and try to implement something similar or extract the clidocgen to a common /library repo?

[johnstcn]

It probably makes sense to use coder/serpent here as what I'm seeing is essentially a reimplementation of a subset of its existing functionality.

[mtojek]

I believe you are heading in the right direction. Most of my comments pertain to adhering to the Go convention, but the overall concept is correct.

I suggest narrowing the scope of this PR to solely adopting coder/serpent (what you have now) and then preparing a follow-up to implement clidocgen. What do you think?

Marcin is going to be on PTO next week

[BrunoQuaresma]

@mafredri @johnstcn @dannykopping I think this PR is in a good state to get merged. Here are some followups from the reviews:

• Test CLI output against a golden file
. Reference - Issue
• Test to validate that envs given previously will still produce the same options now in Serpent
. Reference - Issue
•  Keep the comments on the struct and instead go generate the serpent descriptions from those. Reference - Issue
• Warn if clean up does not work when removing Docker config file. Reference - Issue

I'm planning to create an issue + PR for each one of them after merging #147.

[dannykopping]

LGTM

[johnstcn]