feat: control default services via env vars

Read default postgres compose service name and default django compose
service name from env variables.

AHAB_POSTGRES_CONTAINER - defaults to `appserver`
AHAB_DJANGO_CONATINER - defaults to `postgresdb`

Default values are there for 100% backwards compatibility.

src/scripts/docker_compose.rs

@@ -12,8 +12,10 @@ pub fn down() -> Result<()> {
 }
 
 pub fn exec(rest: &[String]) -> Result<()> {
+    let container = std::env::var("AHAB_DJANGO_CONTAINER").unwrap_or("appserver".to_string());
     CommandBuilder::docker_compose()
-        .args("exec appserver")
+        .args("exec")
+        .args(&container)
         .args(rest)
         .exec()
 }