`Dockerfile.packages` placed build artifacts in `/optimism`, whereas all of our other Dockerfiles place build artifactrs in `/opt/optimism`. As a result, downstream consumers of the packages Dockerfile cannot use images created by our other Dockerfiles. This is a problem for the nightly environment, since it relies upon the `/opt/optimism` structure to customize container entrypoints.

This PR fixes the problem by having `Dockerfile.packages` put build artifacts in `/opt/optimism` like everything else.