Use DOCKMV_TOKEN environment variable for authentication token

- Remove --token=auto flag from docker-compose
- Default token flag to $DOCKMV_TOKEN environment variable instead of empty string
- Update help text to document environment variable default

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -27,7 +27,6 @@ services:
     command:
       - serve
       - --addr=0.0.0.0:8080
-      - --token=auto
 
 volumes:
   migrate-data:

main.go

@@ -81,7 +81,7 @@ Run "dockmv serve -h" for the server flags.
 func serve(args []string) error {
 	fs := flag.NewFlagSet("serve", flag.ContinueOnError)
 	addr := fs.String("addr", "127.0.0.1:8080", "address to listen on; use 0.0.0.0:8080 to expose it on the network")
-	token := fs.String("token", "", "require this token on every request; \"auto\" generates one")
+	token := fs.String("token", os.Getenv("DOCKMV_TOKEN"), "require this token on every request; \"auto\" generates one (default: $DOCKMV_TOKEN)")
 	dataDir := fs.String("data-dir", defaultDataDir(), "directory for connections and trusted host keys")
 	pkgDir := fs.String("package-dir", "", "directory for migration packages (default <data-dir>/packages)")
 	dockerHost := fs.String("docker-host", "", "source docker daemon (default: the DOCKER_HOST environment)")