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

Using a read-only file system results in the following error at the moment:

Failed to create directory '/root/.raku/short' with mode '0o777': Failed to mkdir: No such file or directory
  in any statement_control at /usr/bin/../share/perl6/lib/Perl6/Grammar.moarvm line 1
  in sub MAIN at /usr/share/perl6/vendor/bin/prove6 line 3
  in block <unit> at /usr/share/perl6/vendor/bin/prove6 line 1