I have installed Jenkins from https://hub.docker.com/r/jenkins/jenkins. After creating my user and adding the DSL plugin, when I try to create a DSL script the interface does not show the code box.
I have tried to run the docker image attaching a volume so I can use file system files... but I also didn't manage to make that work either... I'm running out of ideas on how to proceed. All the references that I used show the DSL script box after the installation without a problem.
I had something similar happen to me. This text area seems a bit buggy, for me it was always collapsed. There is a handle in the middle which allows you to change the box size. If you can't see it then this might be a different problem (maybe some browser plugin?).
Also might try clicking between Use the provided DSL script and Look on Filesystem, I vaguely remember this handle showed up after I did that. Sometimes just saving the job (with the JOB DSL step added to the job) helps.
A similar issue happened to me, it is happening in the newer versions. I installed an old version of Jenkins - that old version didn't have this problem
