Add 'git' apt dependency to php Dockerfile

[noisysocks]

Adds git to the php images that have composer installed. git is required by composer in some circumstances, for example when running composer install --prefer-source.

By doing this, I'm aiming to fix the issue described in WordPress/gutenberg#21118 (comment).

[noisysocks]

I tested this in my local wordpress-develop by:

1. Setting php.image in docker-compose.yml to:

   image: docker.pkg.github.com/wordpress/wpdev-docker-images/php:${LOCAL_PHP-latest}-23

2. npm run env:start

3. npm run env:install

4. docker-compose run php composer install --prefer-source

I then confirmed that http://localhost:888/wp-admin/ still loaded and that vendor had been created by composer.

[peterwilsoncc]

Go, git it done.

[pento]

cc @getsource, since we discussed this in #22.