Add missing git executable to backend/Dockerfile

[aspiers]

The node:10-alpine image is missing git, so the docker-compose up --build fails:

Step 4/8 : RUN npm install
 ---> Running in 81dc09d122ed
npm WARN backend@1.0.0 No repository field.

npm ERR! code ENOENT
npm ERR! syscall spawn git
npm ERR! path git
npm ERR! errno ENOENT
npm ERR! enoent Error while executing:
npm ERR! enoent undefined ls-remote -h -t ssh://git@github.com/rxaviers/assertion.git
npm ERR! enoent
npm ERR! enoent
npm ERR! enoent spawn git ENOENT
npm ERR! enoent This is related to npm not being able to find a file.
npm ERR! enoent

I'm not sure how this ever worked before, but the fix is simple, as explained here:

nodejs/docker-node#586

[graup]

This definitely worked before. We don't use any git-style dependencies. I guess a dependency somewhere up the npm module chain changed.
Anyway, thanks for the fix!