I was having the same issue as @Lyraik where the server isn't downloading and installing the .jar file - got round it by essentially downloading the server files onto a windows VM and copied a downloaded version of the forge jar install from the official forge website in. Once that was in the folder I started the server up to install all the files required before copying them into the docker container folder. Bit of a chew on but at least the server is up and running for now.