Expose hard-coded URLs in OpenShift Dockerfiles as Builder ARGs

... Using the example above but a different ENV specification you can create more useful interactions between ARG and ENV instructions:

1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0}
4 RUN echo $CONT_IMG_VER

Unlike an ARG instruction, ENV values are always persisted in the built image. Consider a docker build without the --build-arg flag:

$ docker build Dockerfile

Using this Dockerfile example, CONT_IMG_VER is still persisted in the image but its value would be v1.0.0 as it is the default set in line 3 by the ENV instruction.
The variable expansion technique in this example allows you to pass arguments from the command line and persist them in the final image by leveraging the ENV instruction. Variable expansion is only supported for a limited set of Dockerfile instructions.