Evaluate bash expression in Dockerfile

Question

I'm trying read application version from a file VERSION such that echo -n 1.0.0 > VERSION, and store the version number in an environment variable, lets say, VERSION. My Dockerfile

FROM debian
WORKDIR /app
COPY VERSION .
ENV VERSION $(cat VERSION)
# I'd like to use the version number in later steps like
RUN apt update && apt install -y curl
RUN curl path/to/executable-${VERSION}

env | grep VERSION returns:

VERSION=$(cat VERSION)

I want

VERSION=1.0.0

Answer 1

ENV does not interpolate variables.

How about this:

FROM debian
WORKDIR /app
COPY VERSION version-file
RUN echo "export VERSION=$(cat version-file)" >> /root/.bashrc
RUN apt update && apt install -y curl
RUN curl path/to/executable-${VERSION}

This uses a RUN step to add an export command to .bashrc file. the export command adds the VERSION env var.