Add POSTGRES_EXTRA_OPTS env variable to postgres-backup-s3

Add optional env variable that can be used to pass extra options
to pg_dump, such as restrict the set of tables dumped.

postgres-backup-s3/Dockerfile

@@ -9,6 +9,7 @@ ENV POSTGRES_HOST **None**
 ENV POSTGRES_PORT 5432
 ENV POSTGRES_USER **None**
 ENV POSTGRES_PASSWORD **None**
+ENV POSTGRES_EXTRA_OPTS ''
 ENV S3_ACCESS_KEY_ID **None**
 ENV S3_SECRET_ACCESS_KEY **None**
 ENV S3_BUCKET **None**

postgres-backup-s3/backup.sh

@@ -49,7 +49,7 @@ export AWS_SECRET_ACCESS_KEY=$S3_SECRET_ACCESS_KEY
 export AWS_DEFAULT_REGION=$S3_REGION
 
 export PGPASSWORD=$POSTGRES_PASSWORD
-POSTGRES_HOST_OPTS="-h $POSTGRES_HOST -p $POSTGRES_PORT -U $POSTGRES_USER"
+POSTGRES_HOST_OPTS="-h $POSTGRES_HOST -p $POSTGRES_PORT -U $POSTGRES_USER $POSTGRES_EXTRA_OPTS"
 
 echo "Creating dump of ${POSTGRES_DATABASE} database from ${POSTGRES_HOST}..."