Storing BYOC image definitions for re-building images

[xclayl]

I'm loving the idea of BYOC images. One feature I'd like to see is the ability to store the image definition in my appveyor account (in a *.yml file?).

1. I'd like all co-workers to be able to see the image definition & be able to add to it.
2. I'd like it easy to update (for example add the .Net Core to an existing image). Starting from scratch again means I could forget something and break the build.

"Assisted Setup" is great. I just want to keep the definition around for next time.

One option for me might be to create a Docker image with the software I want. However this isn't practical. The build image would include nodejs, Java, .Net, SQL Server and Octo. Creating a single Docker image with all this is beyond my abilities.

[xclayl]

This is probably in the wrong place. Posted here instead: appveyor/ci#3152