feat: use renovate/yarn as base image

[viceice]

This pr uses renovate/yarn as base image.

So i removed node, npm and yarn.
I also adapted the multistage build to match the slim image

[viceice]

relates to #4708

[rarkins]

I don't think our CI actually tests building so we need to test this manually too

[rarkins]

I'm not sure this achieves anything here, unless we built renovate/yarn badly?

[viceice]

maybe we no longer need to write to this path?

[rarkins]

OK let's remove this section and see if everything is still fine

[viceice]

Have tested it locally against gitlab with docker and yarn manger

[viceice]

maybe we no longer need to write to this path?

[rarkins]

Eventually - maybe v21 - I want to stop bundling npm, yarn and pnpm with Renovate (i.e. remove them as npm dependencies). Until then, we are installing npm and yarn as part of "yarn install" anyway so I'm not sure if we're saving much versus using renovate/node as our base image for publishing instead?

[viceice]

We do not save space for now, but we save some build time and both images have the same base.

And after this it is mode easy to migrate to node v12, because we only need to update base image

When installing nodejs we always have npm installed, bcause it is packaged in nodejs.

[rarkins]

How would it be worse though if the final image was based off renovate/node instead of renovate/yarn? renovate/node should be a subset of renovate/yarn, and we don't run any yarn x commands anyway. And we'd save a little bit of space by not having yarn installed "twice" in the final image (once in renovate/yarn, once by yarn install renovate).

[viceice]

🤔 We can use the renovate/node for final image, but for build we need yarn.

[viceice]

If we use different base images we have to make sure, that both have the same node version.

also yarn only add 2mb (compressed) to the image

[rarkins]

Ok fine, keep as is

[viceice]

So merge to v20?

[rarkins]

Shouldn't we be on python 3 now?