improvement(git): don't require Garden static dir to be a Git repo

[vvagaytsev]

What this PR does / why we need it:
This is a subset of the changes from #4047. The original PR is quite old and requires some conflict resolution.
This PR brings a quick win to improve the usability of Garden.

Which issue(s) this PR fixes:

Fixes #2614
Fixes #4476
Fixes #4477
Fixes #5030

Special notes for your reviewer:
Depends on #5136

[stefreak]

This is great! 🥳

Can you remove git init in the following places in this PR, to verify that garden really works without git in the static directory in our e2e tests:

• garden/support/alpine.Dockerfile

  Line 58 in 6c737a9

  RUN cd /garden/static && git init

• garden/cli/src/build-pkg.ts

  Line 89 in 6c737a9

  await exec("git", ["init"], { cwd: tmpStaticDir })

• garden/support/buster.Dockerfile

  Line 49 in 6c737a9

  RUN cd /garden/static && git init

  
  (Found those quickly scanning the code, but there might be more)

[vvagaytsev]

Good catch, @stefreak! Thank you so much! I'll double-check the rest pieces of the existing code.

[vvagaytsev]

@stefreak I've addressed the comments above in a686596. Please check this again. I haven't found any other places where we init git repo for static dirs.

[stefreak]

Awesome 🥳