Update Dockerfile, adding git

1 job for master in 35 seconds
Name Stage Failure
failed
build Build There has been a runner system failure, please try again
Using Docker executor with image docker:19.03.1 ...
Pulling docker image docker:19.03.1 ...
ERROR: Preparation failed: Error: No such image: docker:19.03.1 (docker.go:205:0s)
Will be retried in 3s ...
Using Docker executor with image docker:19.03.1 ...
Pulling docker image docker:19.03.1 ...
ERROR: Preparation failed: Error: No such image: docker:19.03.1 (docker.go:205:0s)
Will be retried in 3s ...
ERROR: Job failed (system failure): Error: No such image: docker:19.03.1 (docker.go:205:0s)