feature: Support custom workspace images via CLI (`devbox create --image`)

[lengjing]

Problem

Currently, Devboxes only allows configuring custom workspace images globally via Helm values.yaml or through GPU Profiles.

When developers want to quickly test or run an open-source project that already has a pre-built Docker image, they cannot easily pass --image <third-party-image> during devbox create. Running Docker-in-Docker (DinD) inside a devbox is not supported or recommended due to security/privilege risks and missing Docker daemon.

Proposed outcome

Allow developers to specify a custom workspace image when creating a devbox via CLI:

devbox create my-tryout --image ubuntu/nginx:latest

### Alternatives and tradeoffs

_No response_

### Project boundaries

- [x] I considered persistence, security, and Kubernetes portability.

[vicotrbb]

Released in Devboxes v0.5.0.

• Implementation: PR #35
• Release preparation: PR #36
• Documentation: image profiles, configuration, and CLI reference

The released CLI, OCI Helm chart, signed multi-architecture images, SBOMs, provenance, and clean-cluster lifecycle were verified from public artifacts.