fix(devcontainer): correctly parse feature with digest

[maxbrunet]

Hello! 👋

Parsing an image with a digest like ghcr.io/coder/envbuilder:0.2.9@sha256:b51153d8cc1235986707d5f61949884444f2b4c4f64b15b5506cb5484bfa8a93 using NewTag does not work:

repository can only contain the characters `abcdefghijklmnopqrstuvwxyz0123456789_-./`: coder/envbuilder:0.2.9@sha256

ParseReference supports both tag and digest, and produces the same result.

[johnstcn]

Thanks for the contribution! name.ParseReference uses NewTag under the hood (falling back to NewDigest), so this looks fine to me. 👍