Test Dockerfile uses an invalid tag

[kmturley]

In your example Dockerfile you use the dev tag

FROM multiarch/crossbuild:dev

https://github.com/multiarch/crossbuild/blob/master/test/objective-c-hello-world/Dockerfile

However the dev tag does not exist on Docker Hub:
https://hub.docker.com/r/multiarch/crossbuild/tags?page=1&name=dev

Looks like there are two solutions to solve this:

1. Add the tag to Docker Hub
2. Change the test Dockfile to point to a valid tag
   Thanks!