Fix yamllint complaints for docker-compose.yml

While it work it was not technically valid YAML.  Make the YAML valid.

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>

docker-compose.yml

@@ -1,3 +1,4 @@
+---
 version: '2'
 
 services:
@@ -5,6 +6,6 @@ services:
     restart: unless-stopped
     image: sameersbn/apt-cacher-ng:latest
     ports:
-    - "3142:3142"
+      - "3142:3142"
     volumes:
-    - ./apt-cacher-ng:/var/cache/apt-cacher-ng
+      - ./apt-cacher-ng:/var/cache/apt-cacher-ng