I needed an older version of Alpine than what is built by Jenkins and made available on the image repo. I was surprised to find 3.12 is the oldest available - I’m pretty sure I’ve used older than that before.
Is there a retention policy in place where older images are dropped? If so, I couldn’t find any mention of this anywhere.
I was able to use the distrobuilder tool to rebuild older versions of Alpine using the ci image yaml. It took me a few minutes to work out the additional release parameter required, It would be useful if the documentation page explains this - I suspect building images using ci image yaml is a popular usage pattern.
Other than that, it was very easy to build an image with distrobuilder 
but it would have been nice if older image versions remained available on the LXD image server.