This github discussion was found. It seems that it is allowed to build a docker image while a certain ENV does not have been set.
How to ensure that the docker build fails if an ENV has not been set?
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An empty of non existent environment variable is the same thing, there's no reason Docker do not allow you to pass an empty variable as it could be a valid use case.
It's up to your responsibility to ensure needed environment variables are there and give an error message back when not.
Something along the line of:
RUN 'if [ -z "$AN_ENV_VAR" ]; then echo 'Environment variable AN_ENV_VAR must be specified. Exiting.'; exit 1; fi'
You can play with the exit code to automate some actions on failure around your
docker build command.