Maybe with a shell script...

Dockerfile

@@ -4,7 +4,7 @@ ARG PORT=9001
 
 ## mongo build stage
 FROM mongo:$MONGO_VERSION AS database
-COPY mongo-init.js /docker-entrypoint-initdb.d
+COPY mongo-init.sh /docker-entrypoint-initdb.d
 
 ## stage one, build the service
 FROM node:20-alpine AS build

mongo-init.sh

@@ -0,0 +1,14 @@
+set -e
+
+mongo <<EOF
+use $MONGO_INITDB_DATABASE
+
+db.createUser({
+  user: '$DB_USERNAME',
+  pwd: '$DB_PASSWORD',
+  roles: [{
+    role: 'readWrite',
+    db: '$MONGO_INITDB_DATABASE'
+  }]
+})
+EOF