Use base images

[scottyeager]

Currently most or all of the images include sshd along with an init script to set the authorized key and a zinit unit file to run both the script and daemon. This means that the implementations can diverge, and even in the best case that they are consistent, its difficult to maintain and implement a change across all images such as for #165.

Edit: removed original plan in favor of proposal outlined below.

[scottyeager]

Depending on the route we take for host keys, shared files among images could be reduced to just the unit file that launches sshd, thus making this issue less relevant.

Edit: while host key handling will be done by Zos/cloud-container, using base images to factor out common code and get it right once is still worthwhile.

[scottyeager]

After working a bit on updating all images to mitigate the host keys issue, I've generated some additional insights:

• SSH setup is distro independent. Ubuntu/Debian require the same handling for host keys, but Alpine does not, for example
• The vast majority of our images use FROM ubuntu:*. Replacing these with our Ubuntu base variant can eliminate boilerplate and ensure that SSH is setup correctly in all images, using a normal Docker workflow
• Different versions of our Ubuntu base image, like 20.04 and 22.04 can easily be generated from a single Dockerfile, using a VAR

So my updated proposal is as follows:

1. Merge base Ubuntu images (and Debian?) into a single folder and Dockerfile which is used to generate all of the different tags we supply as official images
2. Make all ubuntu:* based solutions derive from our base Ubuntu images, and remove boilerplate which has become duplicated
3. For remaining images which still Ubuntu/Debian based but don't derive from ubuntu:* (just debian and wordpress?), link in the boilerplate files as originally suggested
4. Evaluate if any other images need the same treatment (CentOS)

[scottyeager]

I've implemented the ideas from my last comment in this repo (keeping it separate to avoid package clutter).

A couple more insights from this work:

• Our existing images have a lot of bloat. Just by taking some steps like not installing recommended packages and removing the package lists as recommended in the Docker docs, I've managed to reduce image sizes by half in our generic Ubuntu images and also Presearch
• A number of the images use Docker on Ubuntu, so it makes sense we also build a base image that provides this (which comes FROM one of our Ubuntu images built from the deb/apt base Dockerfile)
• We can take a lesson from Alpine and put busybox inside the base images so it's possible to do a quick ping or ip a without needing to download more packages (some images are shipping utilities like ping for solutions that users will likely never login to). busybox provides wget, which images are typically using to download zinit, while being smaller than wget, so it's a net win.