Add tini to DockerFile

Hi :wave:
### Is your proposal related to a problem?

in the Prefect community, [several people have experienced zombie processes](https://github.com/PrefectHQ/server/issues/335) appearing in the hasura/graphql-engine:v2.1.1 container.

### Describe the solution you'd like

Add `tini` ENTRYPOINT to the hasura DockerFile.

### Describe alternatives you've considered

Users will need to `docker run --init hasura/graphql-engine` if the image does not already have an init.

### If the feature is approved, would you be willing to submit a PR?
I'm afraid I won't be able to dedicate time to this.

Start by locating the Hasura Dockerfile and inspecting how the image currently starts its container process. Check the image build and runtime behavior; done means the resulting container uses tini as its init entrypoint, avoiding the need for users to pass Docker's --init option.

Written by the indexing model from the issue text.