Allow postgres data volume to be overridden with an envvar

This makes it easier to use local database dumps

The compose file version was bumped to 2.1 in order to add support
for ${VARIABLE:-default}

https://docs.docker.com/compose/compose-file/#variable-substitution

[skip ci]

docker-compose.override.yml

@@ -1,5 +1,5 @@
 
-version: '2'
+version: '2.1'
 
 services:
 

docker-compose.ubuntu.yml

@@ -1,5 +1,5 @@
 
-version: '2'
+version: '2.1'
 
 services:
 

docker-compose.yml

@@ -1,6 +1,6 @@
 # Reference README-docker-compose.md for instructions.
 
-version: '2'
+version: '2.1'
 
 volumes:
   postgres_data_vol:
@@ -63,7 +63,7 @@ services:
         sed -i -e 's/max_connections.*/max_connections = 5000/' /var/lib/postgresql/data/postgresql.conf
         sed -i -e 's/#log_min_duration_statement = .*/log_min_duration_statement = 0/' /var/lib/postgresql/data/postgresql.conf
     volumes:
-      - postgres_data_vol:/var/lib/postgresql/data/
+      - "${POSTGRES_DATA_VOL:-postgres_data_vol}:/var/lib/postgresql/data/"
     stdin_open: true
 
   tokumx: