Use Node.js v14 for Docker image, not the latest one (v15)

[Tenzer]

This fixes building of the Docker image, which otherwise fails because the node-canvas dependency doesn't have a prebuilt version available for Node.js v15, and the image doesn't have the necessary system-level dependencies to
build it from source.

Using Node.js v14 (the latest LTS version) fixes this as we then can use the prebuilt node-canvas binary.

As a bonus I have also added clearing of the NPM cache after installing packages, this will decrease the final image size slightly.

[Tenzer]

I believe this is the reason the Docker image at https://hub.docker.com/r/assetgraph/assetgraph-builder hasn't been updated for two years and is stuck on assetgraph-builder version 6.4.3.

[papandreou]

Thanks, Jeppe!

I haven't really used the docker image myself -- it doesn't look like it's wired up to build and push those images on CI? How would you usually do that?

[Tenzer]

The easiest way to do it is probably to set up an automated build in Docker Hub. It will then trigger a new build every time a change is merged into the repository: https://docs.docker.com/docker-hub/builds/. You don't need to build the image yourself, Docker Hub has a build system to do that for you.

[papandreou]

Maybe you can help with that? 🤗

[Tenzer]

I'm willing to give it a try, but I would then need some permissions both on Docker Hub and GitHub to get that configured, I think.

[papandreou]

Cool! I've invited you to the github org now, but I'm not sure how we're set up on Docker Hub. Seems like my Docker Hub user doesn't have access to anything related to assetgraph.

@Munter, do you know how it's wired up?