Docker set ENV based on if-else

Question

I have a situation where I need to set an ENV based on runtime condition like thus:

RUN if [ "$RUNTIME" = "prod" ] then VARIABLE="Some Production URL"; else VARIABLE="Some QA URL"; fi;
ENV={VARIABLE}

Been looking at different solutions but none of them seem to be panning out (for example the basic one where VARIABLE is lost when RUN exits). What would be an elegant way to achieve this?

Answer 1

The literal conditional execution can be achieved with multistage build and ONBUILD.

ARG mode=prod

FROM alpine as img_prod
ONBUILD ENV ELASTICSEARH_URL=whatever/for/prod

FROM alpine as img_qa
ONBUILD ENV ELASTICSEARH_URL=whatever/for/qa

FROM img_${mode}
...

Then you build with docker build --build-arg mode=qa .