Restore controlability with environment variables (#1364)

Since PR #922 the contents of `.env` are included in `Makefile` in order for `make` to by aligned with the `docker-compose` settings.

```
# Make all .env variables available for make targets
include .env
```

The down-side of employing the `include` mechanism is that the settings  in `.env` now take higher precedence than the shell environment variables. As a result, controlling the OpenMapTiles flow because more difficult. For example, tests for `DIFF_MODE=true` had to modify the contents of `.env` in order to work:
b0e7f7884c/.github/workflows/integrity.yml (L45-L47) and
b0e7f7884c/Makefile (L629-L630)

Users were also faced with similar difficulties.

This PR restores the ability to control `make` and `quickstart.sh` using environment variable while keeping the use of the `.env` at a lower priority.

The result is restoring the ability to easily adjust the OpenMapTiles flow using environment variables, such as:
```
PGPORT=54321 DIFF_MODE=true ./quickstart.sh monaco
```

#### Notes: 
1. This PR depends on #1363
2. This PR includes some clean-up of `Makefile`

docker-compose.yml

@@ -54,6 +54,7 @@ services:
       PGUSER: ${PGUSER:-openmaptiles}
       PGPASSWORD: ${PGPASSWORD:-openmaptiles}
       PGPORT: ${PGPORT:-5432}
+      MBTILES_FILE: ${MBTILES_FILE}
     networks:
       - postgres
     volumes:
@@ -130,6 +131,8 @@ services:
     command:
       - --port
       - "${TPORT:-8080}"
+      - --mbtiles
+      - "${MBTILES_FILE}"
     ports:
       - "${TPORT:-8080}:${TPORT:-8080}"
     volumes: