sbt-native-packager-docker-issue-1304

Original bug report at sbt/sbt-native-packager#1304

To reproduce the bug:

git clone https://github.com/hackcave/sbt-native-packager-docker-issue-1304.git
cd sbt-native-packager-docker-issue-1304 && sbt docker:publishLocal

It fails with this error:

chmod: cannot access 'opt/bin/docker/entrypoint.sh': No such file or directory
The command 'chmod u+x,g+x opt/bin/docker/entrypoint.sh' returned a non-zero code: 1