containers: Make config path configurable

Container configuration directory can be set in the .env file which is
automatically used by docker-compose.  The default value is the one used
previously (containers/config).
The HACKING.md guide is updated to describe how to set this and how it
affects other commands.

distribution/.env

@@ -1 +1,2 @@
 COMPOSE_PROJECT_NAME=osbuild
+CONTAINER_CONFIG_DIR=../containers/config

distribution/docker-compose.yml

@@ -6,7 +6,7 @@ services:
       context: ..
       dockerfile: ./distribution/Dockerfile-ubi
     volumes:
-      - ../containers/config:/etc/osbuild-composer
+      - ${CONTAINER_CONFIG_DIR}/:/etc/osbuild-composer
     networks:
       net:
         ipv4_address: 172.30.0.10
@@ -18,7 +18,7 @@ services:
     # override the entrypoint to specify composer hostname and port
     entrypoint: /usr/libexec/osbuild-composer/osbuild-worker composer:8700
     volumes:
-      - ../containers/config:/etc/osbuild-composer
+      - ${CONTAINER_CONFIG_DIR}/:/etc/osbuild-composer
     environment:
       - CACHE_DIRECTORY=/var/cache/osbuild-worker
     privileged: true