* exit codes of failed RUN commands may now be propagated to the caller by using the --propagate-exit-code flag
* removed exit code propagation flag; the exit function now unwraps the error and looks for an exit code
* added integration test for exit code propagation
* added trailing new line in test Dockerfile_exit_code_propagation; tidying up README.md removing all traces of error propagation flag
* moved exit code propagation test to integration_test.go; moved exit code propagation test files to 'testdata' to avoid interference with other integration tests
Benjamin Krenn