Containers in linux are primarily a shipping method (as Docker themselves try to inform you with the visual of a shipping container).
Just like real shipping containers, dangerous things inside can leak out - the isolation is not foolproof by any means, in fact if someone has the express wish of violating the isolation boundary it's barely an inconvenience.
Just like real shipping containers, dangerous things inside can leak out - the isolation is not foolproof by any means, in fact if someone has the express wish of violating the isolation boundary it's barely an inconvenience.