Fix: Docker build failing on newer node:lts-alpine3.21 version

[cotton105]

Update the Dockerfile to specifically use the node:lts-alpine3.20 image.

I was having issues building locally, and found that building with the most recent node image release (lts-alpine3.21) does not succeed: docker-build.log

Building was working on one of my machines but not the other, and I realised that the successful build came from the old 3.20 image that I already had on the working machine. The Dockerfile is using the lts-alpine tag, which has been updated upstream to refer to lts-alpine3.21.

I don't know whether it would be preferable to use the newer 3.21 version and fix the dependencies, but pinning to 3.20 has fixed the build on my end. It may be a good idea anyway to pin a specific version, to prevent this from happening again.

Checklist

• Documentation is changed or added (if applicable)
• Commit message follows the commit guidelines

[atlanna]

Thank you for opening this pull request.
A maintainer will get by as soon as practical to review the changes.