ci: add .git to the Docker container

Because commitlint needs a .git repository to lint all commits on a branch, we're adding it to the
Docker container.

re #121
8 jobs for conventional_commits in 5 minutes and 2 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #9551
build

00:00:22

 
  Test
passed #9554
lint:commits

00:00:12

passed #9553
lint:js

00:00:30

passed #9552
lint:style

00:00:14

passed #9555
test

00:03:45

 
  Review
passed #9556
review:ui

00:00:12

passed #9557
allowed to fail manual
stop_review:ui

00:00:16

 
  Demo
manual #9558
allowed to fail manual
demo:ui