Hi Rujin Shi,
For unlicensed applications, only 6 concurrent users are allowed. You need to get licence key from Mendix Support, and during the Docker Run, the license key should be included.
https://github.com/mendix/docker-mendix-buildpack#current-limitations
Sample :
docker run -it \
-p 8080:80 \
-e ADMIN_PASSWORD=Password1! \
-e DATABASE_ENDPOINT=postgres://mendix:mendix@172.17.0.2:5432/mendix \
-e LICENSE_ID=<UUID> \
-e LICENSE_KEY=<LICENSE_KEY> \
mendix/mendix-buildpack:v1.2
Thanks!