Add git binary to Docker image

[ossinkine]

The documentation says it can be used in the CI the simple way:

# Inside the .circleci/config.yml file:
version: 2
jobs:
  build:
    docker:
      - image: kamilkisiela/graphql-inspector
    steps:
      - checkout
      - run:
          name: GraphQL Inspector
          command: 'graphql-inspector diff "git:origin/master:./schema.graphql" "schema.graphql"'

But this example fails with an error git: command not found (I've tested on Gitlab CI, not Circle CI, but it doesn't matter), so I suggest to install the git binary in the kamilkisiela/graphql-inspector image