docker run -dp 3389:3389 --restart=always swined/rdp
run the image, find it's id with docker ps, then run docker exec -it <containerId> adduser <username> --ingroup sudo and follow the prompts. the user will have admin access, check out man adduser for more details on configuring user's permissions.
run echo -n <password> | openssl passwd -crypt -stdin and copy the hashed password, that is returned. create a folder, and put there a 'Dockerfile' with the following content:
FROM swined/rdp
RUN useradd -mp <hashedPassword> -s /bin/bash -G sudo <username>
run docker build -t <imageName> . and then run the image as usual, replacing 'swined/rdp' with your custom image name
create a Dockerfile as described earlier, and put a line with RUN apt-get update && apt-get install -y --force-yes <listOfUbuntuPackageNames> && apt-get clean instead of 'RUN useradd ...', or combine both 'useradd' and 'apt-get' if you need both custom users and custom software