Switch to authorized keys for image login

[alexhulbert]

Currently, devtools images can't be safely used in situations where they are accessible over the public internet. This is for a few reasons:

• The devtools images allow password-based root login
• The devtools images allow logging in over the serial console

The current L2 merge branch works around this by creating custom modules that disable these two features. However, this works around functionality that shouldn't be enabled by default in the first place.

This PR disables password-based root login and the serial console in devtools images (there is no passwd entry for root, so it will never allow any login even if someone somehow managed to get a login prompt). This means that the only way to log in to a devtools image is via injecting an authorized_keys file into the image.

To make this injection easier, the PR also allows you to add an authorized_keys file to mkosi.profiles/devtools. It will inject this file for you automatically into dev builds. This file is gitignored too, so you don't need to worry about committing it.

To enable the original serial console / password-based login functionality, you can add SERIAL_CONSOLE=true to the end of a make build-dev command.

[0x416e746f6e]

To make this injection easier, the PR also allows you to add an authorized_keys file to mkosi.profiles/devtools

I think this breaks reproducibility of the build.

[alexhulbert]

@0x416e746f6e It doesn't break reproducibility since it only injects the key into dev images, similar to how the Yocto tooling worked.

[0x416e746f6e]

It doesn't break reproducibility since it only injects the key into dev images, similar to how the Yocto tooling worked.

i.d.k. whether how yocto worked holds an argument here. but dev images are also images, and injecting arbitrary files into them at build-time will cause them to be different from one another, hence will break reproducibility. throwing that away just for the sake of allowing ssh access to the dev VM is too much i.m.o.

there are multiple ways to allow SSH access into dev VMs while still keeping the images reproducibly buildable. l2 is using one of them, but there are definitely others available too.