Note that I had to edit the ~/.dput.cf file and change http to https for the dput command to work.
# will build the source and binary package for amd64
docker build -t build_stlcmd_deb .
# runs bash, cd /tmp where the packages are built
docker run -ti build_stlcmd_deb
# in another terminal
docker ps
docker cp <private key> <container id>:/tmp
# in container terminal
gpg --import /tmp/<private key>
cd /tmp
debsign -k <pgp id> stlcmd_1.0-1_amd64.changes
dput mentors stlcmd_1.0-1_amd64.changes
exit