Checkout the repository with git

This changes the build process even further. Instead
f using `wget` to fetch the current code, `git` is used.
This allows for faster switching between branches,
because only the differences between them have to be
fetched from the server.

But the main advantage is that the build cache can
finally be used as designed by Docker. Repetitive
builds are very fast now. This is also true between
branches and tags, as long as the `requirements.txt`
file doesn't change.

build-all.sh

@@ -18,9 +18,6 @@ fi
 
 ERROR=0
 
-# Don't build if not on `master` and don't build if on a pull request,
-# but build when DEBUG is not empty
-
 for BUILD in "${BUILDS[@]}"; do
   echo "🛠 Building '$BUILD' from '$DOCKERFILE'"
   case $BUILD in