I was shocked when I read the instructions and realized I had to build a secondary container.
On top of this, there are some keycloak options (parameters and env vars) which are valid only at the time of compiling a new container, where others are valid only at runtime
Anyway, they changed the default behavior a few releases ago. In the past you had to pass --auto-build to avoid having to build, but now the default behavior is to auto build. If you instead want to optimize you have to do extra steps: https://www.keycloak.org/server/configuration