My dockers ENV YARN_VERSION does not match the actual version

Question

Inside my docker container, the environment variable for my yarn version seems to be completely off. I am using Yarn 2, configured in the package.json. It works well, but I was confused when I stumbled over the ENV value.

What is the reason and might it have any side-effects?

package.json

{
  "name": "my-project",
  "type": "module",
  "scripts": {
    "watch": "node --watch src/app.js",
    "start": "node src/app.js",
  },
  "packageManager": "[email protected]"
}

Dockerfile:

FROM node:21.6-alpine3.19
ENV NODE_ENV=production
RUN corepack enable
USER node
WORKDIR /home/node/app
COPY --chown=node:node . .
RUN yarn install --immutable
CMD ["yarn", "start"]

checking the actual version:

~/app $ yarn -v
4.0.2

checking the env:

~/app $ echo $YARN_VERSION
1.22.19

Answer 1

The reason for this discrepancy is that the YARN_VERSION environment variable is defined in the base image.

$ docker run -it node:21.6-alpine3.19 /bin/sh
/ # yarn -v
1.22.19
/ # echo $YARN_VERSION
1.22.19

In your derived image you install another version of yarn. If you want then environment variable to be consistent then you can update it.

├── Dockerfile
├── entrypoint.sh
├── package.json
└── src
    └── app.js

Dockerfile

FROM node:21.6-alpine3.19

ENV NODE_ENV=production
RUN corepack enable

WORKDIR /app

COPY . .

RUN yarn install && \
    chmod +x ./entrypoint.sh

ENTRYPOINT ["./entrypoint.sh"]

CMD ["yarn", "start"]

entrypoint.sh which will update YARN_VERSION.

#!/bin/sh

export YARN_VERSION=$(yarn -v)
exec "$@"

package.json as supplied in original question.

src/app.js a little script with a greeting and the value of YARN_VERSION.

console.log("Hello, World! from Yarn "+process.env.YARN_VERSION);