Set a default retention policy using env variables and docker

Is that possible to set a default retention policy on the db created with the `INFLUXDB_DB` env variable with docker?

Here is my `docker-compose.yml` file:

```yaml
grafana-db:
container_name: grafana-db
image: influxdb:1.7-alpine
volumes:
- my_data:/var/lib/influxdb
environment:
- INFLUXDB_DB=my_db
- INFLUXDB_USER=my_user
- INFLUXDB_USER_PASSWORD=my_password
restart: unless-stopped
user: "1000"
```

That would be nice to be able to do that without having to write a custom init script and so a `Dockerfile` only for this.

Start with the Docker configuration that handles INFLUXDB_DB and review the provided docker-compose.yml for how the database is created. Define how a default retention policy would be supplied through environment variables or Docker configuration, and verify that a database created from INFLUXDB_DB receives it without a custom init script.

Written by the indexing model from the issue text.