Expand ARG value in CMD [Dockerfile] - docker

Question

I'm passing a build argument into: docker build --build-arg RUNTIME=test
In my Dockerfile I want to use the argument's value in the CMD:
CMD ["npm", "run", "start:${RUNTIME}"]
Doing so results in this error: npm ERR! missing script: start:${RUNTIME} - it's not expanding the variable
I read through this post: Use environment variables in CMD
So I tried doing: CMD ["sh", "-c", "npm run start:${RUNTIME}"] - I end up with this error: /bin/sh: [sh,: not found
Both errors occur when I run the built container.
I'm using the node alpine image as a base. Anyone have ideas how to get the argument value to expand within CMD? Thanks in advance!
full Dockerfile:
FROM node:10.15.0-alpine as builder
ARG RUNTIME_ENV=test
RUN mkdir -p /usr/app
WORKDIR /usr/app
COPY . .
RUN npm ci
RUN npm run build
FROM node:10.15.0-alpine
COPY --from=builder /usr/app/.npmrc /usr/app/package*.json /usr/app/server.js ./
COPY --from=builder /usr/app/config ./config
COPY --from=builder /usr/app/build ./build
RUN npm ci --only=production
EXPOSE 3000
CMD ["npm", "run", "start:${RUNTIME_ENV}"]
Update:
Just for clarity there were two problems I was running into.
1. The problem as described by Samuel P.
2. ENV values are not carried between containers (multi-stage)
Here's the working Dockerfile where I'm able to expand environment variables in CMD:
# Here we set the build-arg as an environment variable.
# Setting this in the base image allows each build stage to access it
FROM node:10.15.0-alpine as base
ARG ENV
ENV RUNTIME_ENV=${ENV}
FROM base as builder
RUN mkdir -p /usr/app
WORKDIR /usr/app
COPY . .
RUN npm ci && npm run build
FROM base
COPY --from=builder /usr/app/.npmrc /usr/app/package*.json /usr/app/server.js ./
COPY --from=builder /usr/app/config ./config
COPY --from=builder /usr/app/build ./build
RUN npm ci --only=production
EXPOSE 3000
CMD npm run start:${RUNTIME_ENV}

Answer 1

The problem here is that ARG params are available only during image build.
The ARG instruction defines a variable that users can pass at build-time to the builder with the docker build command using the --build-arg <varname>=<value> flag.
https://docs.docker.com/engine/reference/builder/#arg
CMD is executed at container startup where ARG variables aren't available anymore.
ENV variables are available during build and also in the container:
The environment variables set using ENV will persist when a container is run from the resulting image.
https://docs.docker.com/engine/reference/builder/#env
To solve your problem you should transfer the ARG variable to an ENV variable.
add the following line before your CMD:
ENV RUNTIME_ENV ${RUNTIME_ENV}
If you want to provide a default value you can use the following:
ENV RUNTIME_ENV ${RUNTIME_ENV:default_value}
Here are some more details about the usage of ARG and ENV from the docker docs.