Last time I checked, when I did this:
FROM x:latest
ENV foo 'bar'
FROM y:latest
RUN echo "$foo"
then "echo $foo" was empty - is there a way to persist ENV vars across multi-stage builds?
A build argument might work for you in this case. The user won't be able to override it and it won't be available in the container but other than that I think it fits.
FROM alpine
ARG FOO
RUN echo first step FOO is $FOO
FROM alpine
ARG FOO
RUN echo second step FOO is $FOO
To build you need to pass --build-arg with a value.
$ docker build --build-arg FOO=bar .
Step 1/6 : FROM alpine
---> 055936d39205
Step 2/6 : ARG FOO
---> Running in 3f5f18206d06
Removing intermediate container 3f5f18206d06
---> 2b82e4b958f7
Step 3/6 : RUN echo first step FOO is $FOO
---> Running in c0256dfe286d
first step FOO is bar
Removing intermediate container c0256dfe286d
---> 79286b74611f
Step 4/6 : FROM alpine
---> 055936d39205
Step 5/6 : ARG FOO
---> Running in 9fc20546619f
Removing intermediate container 9fc20546619f
---> 30325962d73a
Step 6/6 : RUN echo second step FOO is $FOO
---> Running in a8906382909a
second step FOO is bar
Removing intermediate container a8906382909a
---> 521dbbfa398b
Successfully built 521dbbfa398b