How can I evaluate and assign default value to ARG in dockerfile?

Question

Basically I want to assign a dynamically calculated default value for ARG in my Dockerfile:

FROM node:20-alpine

ARG RELEASE_TAG=$(git describe --tags --always)

# Other commands...

So, far it doesn't interpolate the value of $(git describe --tags --always) and simply assigns this value to the RELEASE_TAG variable.

Is there any way I can achieve this?

Answer 1

You can achieve the expected effect by putting it in RUN :

FROM ubuntu

WORKDIR /app

COPY . .

RUN apt-get update && apt-get install -y git
RUN RELEASE_TAG=$(git describe --tags --always) &&\
    echo npm run build -- --version=$RELEASE_TAG

CMD sleep inf