Add .env setup instructions to Docker Hub docker-compose section

DOCKER_HUB.md

@@ -148,7 +148,21 @@ When a config file is mounted, the `OPENCODE_PROVIDER` auto-config is skipped.
 
 ## Using docker-compose
 
-Create a `docker-compose.yml`:
+Create a `docker-compose.yml` and a `.env` file in the same directory:
+
+```bash
+mkdir opencode-devbox && cd opencode-devbox
+```
+
+`.env` — your secrets and settings (never commit this):
+
+```bash
+ANTHROPIC_API_KEY=sk-ant-...
+GIT_USER_NAME=Your Name
+GIT_USER_EMAIL=you@example.com
+```
+
+`docker-compose.yml`:
 
 ```yaml
 services:
@@ -176,6 +190,8 @@ volumes:
   devbox-data:
 ```
 
+Docker Compose automatically loads `.env` from the same directory as the compose file. The `${VAR}` references are substituted with values from `.env`.
+
 Then:
 
 ```bash