Image names can't contain common version symbols (such as `.`)

Our rules for resource names are roughly those of DNS labels, as specified in RFC 1035 (this decision is somewhat documented in RFD 4). This is useful for resources which may eventually get DNS-based resources, such as instance names.

Image need to have unique names, and so it is common to include a version number of the image in a name. For instance, the latest Alpine image is named alpine-standard-3.18.0-x86_64.iso. It would probably make sense to give this image a name like alpine-3.18.0 to distinguish it from other Alpine releases. This is not currently possible.

Users can keep track of the distro and version of the image in separate fields, but it feels very unnatural to have to name the image something like alpine-3-18-0.

We should also consider whether to include other common version symbols (+ and maybe whatever else is allowed in semver). Whether a symbol needs to be urlencoded or not is a useful factor.

Start by reading RFD 4 and RFC 1035, then trace the resource-name validation used for images. Determine which version symbols should be accepted and how URL encoding affects the decision; done means the supported naming rules and relevant validation coverage are agreed and implemented.