I found a workaround for the "no X11 Display error" that seems to work for me on the FTBOceanBlock Container I just set an environment variable with the name DISPLAY and add the value :0.0
After this i got the following error:
§7java.lang.UnsatisfiedLinkError: /usr/local/openjdk-8/lib/amd64/libawt_xawt.so: libXrender.so.1: cannot open shared object file: No such file or directory
I fixed this by manually installing these packages in the container. libxrender1 libxtst6 libxi6
docker exec "container" apt install libxrender1 libxtst6 libxi6 -y
After that everything worked perfectly. I hope this helps.