Problems building docker image for arm64

[rpgoldman]

(For those who, like myself, have an M1 Mac:) It's not possible to build the Docker image for arm64 because of a goof in the way sbt is packaged for Debian. It's actually an architecture-independent package, but apt-get does not recognize it as such. For more details, see sbt/sbt#6614

I have worked around this for now by hacking the following into the Dockerfile, but it's ugly. I'm not sure what's a better solution:

# Workaround because of bug in sbt from Debian.  
# See https://github.com/sbt/sbt/issues/6614
RUN wget https://scala.jfrog.io/artifactory/debian/sbt-1.6.2.deb && \
    apt install ./sbt-1.6.2.deb