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 "$AN_ENV_VAR" ]; then echo 'Environment variable AN_ENV_VAR must be specified. Exiting.'; exit 1; fi'
You can play with the exit code to automate some actions on failure around your
docker build command.
if command construct is very complex for you, you can always simply it like
RUN [ -n "$VAR" ]
This asserts the variable value length is nonzero. If it's empty or not set it'll exit with 1. This is also same as running
RUN test -n "$VAR"
I see this error message on my machine
The command '/bin/sh -c [ -n "$VAR" ]' returned a non-zero code: 1