this simple Dockerfile just adds the pnpm package manager to a base node image which can be set using the NODE_BASE_IMG build arg. There are multiple image variant avaiable. Each one based on a different node image. One is based on node:fermium-alpine, one on node:gallium-alpine and one on node:hydrogen-alpine. For each of those "base-alpine" images, there is also a "slim" and a "regular" base image.
In your Dockerfile use the FROM command to include this image, with the node version tag as this images tag:
FROM ghcr.io/andreas-fahrecker/node-pnpm:{NODE_VERSION_TAG}