how to dynamically set an environment variable to true within a dockerfile - docker

Question

my app has feature flags that i would like to dynamically set to true for my npm build.
essentially i'd like to do something like
COMPILE_ASSETS=true npm build or NEW_EMAILS=true npm build, only dynamically from CI.
i have a CI pipeline that will grab the flag, but am having trouble setting it to the true and running npm in the Dockerfile.
my Dockerfile -
FROM ubuntu:bionic
ARG FEATURE_FLAG
RUN if [ "x$FEATURE_FLAG" = "x" ] ; \
then npm run build ; \
else $FEATURE_FLAG=true npm run build; \
fi
this gets run with --
docker build --no-cache --rm -t testing --build-arg FEATURE_FLAG=my_feature_flag . (i would like to keep this the way it is)
in CI i get
/bin/sh: 1: my_feature_flag=true: not found
i've tried various forms of the else statement --
else export $FEATURE_FLAG=true npm run build; (this actually looks like it works on my mac but fails in CI with export: : bad variable name
else ${FEATURE_FLAG:+$FEATURE_FLAG=true} npm build;
else eval(`$FEATURE_FLAG=true npm build`);
`else env $FEATURE_FLAG=true bash -c 'npm build';
these all fail :(
i've tried reworking the Dockerfile completely and setting the flag to true as an ENV --
ARG FEATURE_FLAG
ENV FF_SET_TRUE=${FEATURE_FLAG:+$FEATURE_FLAG=true}
ENV FF_SET_TRUE=${FF_SET_TRUE:-null}
RUN if [ "$FF_SET_TRUE" = "null" ] ; \
then npm build; \
else $FF_SET_TRUE npm build; \
fi
nothing works! is this simply a bash limitation? is expanding a variable before running a command is not possible?
or is this not possible with Docker?

Answer 1

Did you mean:
FROM ubuntu:bionic
ARG FEATURE_FLAG
RUN set -eux; \
if [ "x$FEATURE_FLAG" == "x" ] ; then \
npm run build ; \
else \
eval $($FEATURE_FLAG=true npm run build); \
fi
You need to wrap your command in eval for the variable to expand based on the ARGS being passed.

Answer 2

this worked!
ARG FEATURE_FLAG
RUN if [ -z "$FEATURE_FLAG" ] ; \
then npm run build ; \
else \
echo setting $FEATURE_FLAG to true; \
env "$FEATURE_FLAG"=true sh -c 'npm run build'; \
fi