Allow for when SSH_AUTH_SOCK is not defined. (#73)

docker-compose.linux.yml

@@ -8,4 +8,4 @@ services:
       SSH_AUTH_SOCK: ${SSH_AUTH_SOCK}
     volumes:
       # Mount SSH agent so we can authenticate against github inside of the IDE container.
-      - ${SSH_AUTH_SOCK}:${SSH_AUTH_SOCK}:Z
+      - ${SSH_AUTH_SOCK:-/dev/null}:${SSH_AUTH_SOCK:-/tmp/null}:Z