Conditional ENV in Dockerfile

Is it possible to conditionally set an ENV variable in a Dockerfile based on the value of a build ARG?

Ex: something like

ARG BUILDVAR=sad
ENV SOMEVAR=if $BUILDVAR -eq "SO"; then echo "hello"; else echo "world"; fi

Update: current usage based on Mario's answer:

ARG BUILD_ENV=prod
ENV NODE_ENV=production
RUN if [ "${BUILD_ENV}" = "test" ]; then export NODE_ENV=development; fi

However, running with --build-arg BUILD_ENV=test and then going onto the host, I still get

docker run -it mycontainer bin/bash
[root@brbqw1231 /]# echo $NODE_ENV
production

Yes, it is possible, but you need to use your build argument as flag. You can use parameter expansion feature of shell to check condition. Here is a proof-of-concept Docker file:

FROM debian:stable
ARG BUILD_DEVELOPMENT
# if --build-arg BUILD_DEVELOPMENT=1, set NODE_ENV to 'development' or set to null otherwise.
ENV NODE_ENV=${BUILD_DEVELOPMENT:+development}
# if NODE_ENV is null, set it to 'production' (or leave as is otherwise).
ENV NODE_ENV=${NODE_ENV:-production}

Testing build:

docker build --rm -t env_prod ./
...
docker run -it env_prod bash
root@2a2c93f80ad3:/# echo $NODE_ENV 
production
root@2a2c93f80ad3:/# exit
docker build --rm -t env_dev --build-arg BUILD_DEVELOPMENT=1 ./
...
docker run -it env_dev bash
root@2db6d7931f34:/# echo $NODE_ENV
development

You cannot run bash code in the Dockerfile directly, but you have to use the RUN command. So, for example, you can change ENV with RUN and export the variable in the if, like below:

ARG BUILDVAR=sad 
RUN if [ "$BUILDVAR" = "SO" ]; \
    then export SOMEVAR=hello; \
    else export SOMEVAR=world; \
    fi 

I didn't try it but should work.

Your logic is actually correct. The problem here is that RUN export ... won't work in a Dockerfile because the export command won't persist across images. Dockerfiles create a temporary container in order to generate the image for it, therefore the environment variables won't exist.

ENV on the other hand as per the documentation states:

The environment variables set using ENV will persist when a container is run from the resulting image.

The only way to do this is during your docker run command when generating the container from your image, and wrap your logic around that:

if [ "${BUILD_ENV}" = "test" ]; then
    docker run -e NODE_ENV=development myimage
else
    docker run myimage
fi

While you can't set conditional ENV variables but you may be able to acomplish what you are after with the RUN command and a null-coalescing environment variable:

RUN node /var/app/current/index.js --env ${BUILD_ENV:-${NODE_ENV:-"development"}}

If we are talking only about environment variable, then just set it with production

ENV NODE_ENV prod

And during container start in development, you may use -e NODE_ENV=dev.

This way image is always built-in production but the local container is launched in development.