Hi 👋

Every image is built for multiple different OS flavors (eg. bookworm, bullseye, focal, jammy etc). Hence, you can pin your image to your choice of OS. Then for semantic versioning, https://github.com/devcontainers/images/blob/main/src/rust/manifest.json#L2 points to the version of the image.

Which version number the -1 part referring to here?

We recommend users to pin to major versions in order to receive bug fixes, security patches, improvements etc.
In case of rust, 1- refers to latest published image, it will continue to receive minor and patch versions automatically.

If we omit the bookworm part, which version does it pull? (and where is this defined).

The default is bookworm, hence you will end up with a bookworm distro. See https://github.com/devcontainers/images/blob/main/src/rust/manifest.json#L9

Moving further, we notice that this image is versioned as 1.0.7, which in turn is built from v0.3.27.

1.0.7 is the version of the image and v0.3.27 is the version of the release. Feel free to ignore release versions because we use that for tracking image releases.

https://github.com/devcontainers/images/tree/main/src/rust/history has release docs published for every image version, this is helpful for comparing changes and keeping track of image contents.

Let us know if anything is unclear, happy to clarify any other doubts.