Before this commit, a user using `crystallang/crystal` docker image (e.g. on CI) was not able to use `shards`
which requires `git` to be installed if the dependencies' source is git-based (GitHub, BitBucket or pure git).

I think `shards` is an essential tool for every Crystal project.

This commit installs `git` in the Docker image you don't have to `apt-get install -y git` over and over :-)