You'll have a hard time finding people who will say you are secure, but there are no known vulnerabilities allowing breaking out of a default-configuration dock

You'll have a hard time finding people who will say you are secure, but there are no known vulnerabilities allowing breaking out of a default-configuration docker container at this time. JessFraz has been running https://contained.af/ https://contained.af/, which is a much more locked-down than the default container, for ages, and no one has broken out and captured the flag yet. If you do some legwork, it's not too bad currently.