feat: new Dockerfile base image

[guidojw]

Summary

I originally put the ember-cli dependencies in de Ember.js group in #372, but found out that ember-cli versions are independent of Ember.js updates and newer CLI versions can still build older Ember.js apps. This PR changes the base image in the Dockerfile to another ember-cli image which has the following pros and cons:

Pros

• It's updated more frequently
• Its Node.js version actually matches most up-to-date ember-cli support (madnificent's (one we use now) doesn't sadly)
• Unblocks chore(deps): update dependency eslint to v8 #399 (due to compatible Node.js version, above point)
• It looks like this can also easily be used in a development environment (I tried in Feature/Allow to run dev server via Docker #235, did not succeed, can try again with this one as base in the future)
• It installs Chrome drivers so makes Dockerfile.test redundant but the overall size is a bit bigger, the bundle size isn't though since the NGINX stage in the Dockerfile still does the same

Cons

• The overall size is a bit bigger

I also used digest pinning for the Docker image versions of ember-cli and NGINX.

Other information

• https://github.com/danlynn/ember-cli

[guidojw]

Tests fail because this needs #411