How to make a build arg mandatory during Docker build?

Question

Is there any way to make a build argument mandatory during docker build? The expected behaviour would be for the build to fail if the argument is missing.

Answer 1

Long time ago I had a need to introduce a required (mandatory) ARG, and for better UX include the check at the beginning:

FROM ubuntu:bionic
ARG MY_ARG
RUN [ -z "$MY_ARG" ] && echo "MY_ARG is required" && exit 1 || true

...

RUN ./use-my-arg.sh

But this busts the build cache for every single layer after the initial MY_ARG, because MY_ARG=VALUE is prepended to every RUN command afterwards.

Whenever I changed MY_ARG it would end up rebuilding the whole image, instead of rerunning the last RUN command only.

To bring caching back, I have changed my build to a multi-staged one:

• The first stage uses MY_ARG and checks it's presence.
• The second stage proceeds as usual and declares ARG MY_ARG right at the end.

FROM alpine:3.11.5
ARG MY_ARG
RUN [ -z "$MY_ARG" ] && echo "MY_ARG is required" && exit 1 || true

FROM ubuntu:bionic
...
ARG MY_ARG
RUN ./use-my-arg.sh

Since ARG MY_ARG in the second stage is declared right before it's used, all the previous steps in that stage are unaffected, thus cache properly.