Support read-only file system

[ErikSchierboom]

When running in Docker, test runners should support being run on a read-only filesystem. The only directories that are writeable are:

• The /opt/test-runner directory
• The /tmp directory
• The input directory
• The output directory

To simulate this behavior, the --read-only flag has to be passed to the two docker run calls at the following locations:

• https://github.com/exercism/scala-test-runner/blob/main/bin/run-in-docker.sh#L36
• https://github.com/exercism/scala-test-runner/blob/main/bin/run-tests-in-docker.sh#L19

I've briefly looked into this, and the main problem seems to be that SBT is trying to write a lock file in the ~/.sbt directory.