Exit Code Propagation (#1655)

* 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

integration/testdata/exit-code-propagation/Dockerfile_exit_code_propagation

@@ -0,0 +1,5 @@
+FROM alpine:latest
+
+RUN exit 42
+
+CMD ["sleep", "1"]