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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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    RUN 'if [ -Z $ENV ]; then exit 1; fi' ? – Tensibai Jul 17 '17 at 9:24
  • Yes, but I would like to see that docker build itself denies it – 030♦ Jul 17 '17 at 9:26
  • Why that ? 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 very valid use case... It's up to your responsibility to ensure needed environment variables are there and give an error message back when not. – Tensibai Jul 17 '17 at 9:30
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    @Tensibai perhaps you could combine both comments and post it as an answer so it could be accepted – 030♦ Jul 19 '17 at 7:09
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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.

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To error out the build, just add the following with your env variable in the docker-compose file.

${ENV_VAR?Variable ENV_VAR not set}

example:

image: consul:${TAG?Variable TAG not set}
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