Dockerfile ENV not replacing in sed - docker

Question

I have set up dockerfile with making changes to config files with sed.
The image is a simple rtmp restreamer.
The lines in dockerfile go like:
RUN sed -i 's/ytkey/${YOUTUBE_KEY}/g' /etc/nginx/nginx.conf &&\
sed -i 's/fbkey/${FACEBOOK_KEY}/g' /etc/nginx/nginx.conf
i set up ENV at the beginning
ENV YOUTUBE_KEY=default FACEBOOK_KEY=default
but after building the image it doesn't replace ytkey or fbkey as default but inputs a string ${YOUTUBE_KEY}.
I have tried running
RUN sed -i 's/ytkey/\${YOUTUBE_KEY}/g' /etc/nginx/nginx.conf &&\
sed -i 's/fbkey/\${FACEBOOK_KEY}/g' /etc/nginx/nginx.conf
But had the same result. Is it a problem with sed (having the env in '') or something else? Is there an alternative to sed that works with dockerfile?
Or maybe i'm doing things completely the wrong way?

Answer 1

You can use double quotation marks around environment variables like this:
RUN sed -i 's/user/'"${YOUTUBE_KEY}"'/g' /etc/nginx/nginx.conf &&\
sed -i 's/pid/'"${FACEBOOK_KEY}"'/g' /etc/nginx/nginx.conf

Answer 2

I realise the question is specifically about sed, but you might also consider using envsubst to do this, especially if you start to use more variables.
It processes stdin to stdout, so in your example you would need to use a temp file strategy, but personally I prefer that approach:
envsubst < /etc/nginx/nginx.conf > /etc/nginx/nginx.conf.tmp
mv /etc/nginx/nginx.conf.tmp /etc/nginx/nginx.conf
For more complex configs, your templates would often be in a different directory anyway, so this wouldn't be necessary.