docker: Add `$USER` env var to docker-compose

This ensures the variable is set and can be used as a default for the
node alias.

docker-compose.yml

@@ -10,6 +10,7 @@ services:
       RUST_LOG: debug
       RAD_PASSPHRASE: seed
       RUST_BACKTRACE: 1
+      USER: ${USER}
     volumes:
       - /mnt/radicle/heartwood:/root/
     init: true
@@ -31,6 +32,7 @@ services:
     environment:
       RUST_LOG: debug
       RUST_BACKTRACE: 1
+      USER: ${USER}
     volumes:
       - /mnt/radicle/heartwood:/root/
     init: true