Investigate timezone handling for shared dev images

[ScottMorris]

Summary

Investigate whether the shared dev image should keep setting the timezone in docker/ci/Dockerfile, or whether timezone preferences should move to downstream devcontainer.json configuration.

Context

The current dev image sets:

ENV TZ=America/Toronto
RUN ln -snf /usr/share/zoneinfo/${TZ} /etc/localtime \
    && echo "${TZ}" > /etc/timezone

That gives the image a system-level timezone default, but it also bakes a local preference into a shared image.

What we know

• devcontainer.json can set TZ through remoteEnv or containerEnv.
• That is a good fit when the goal is editor, terminal, and app-level timezone behaviour inside a devcontainer.
• Setting TZ in devcontainer.json does not rewrite /etc/localtime or /etc/timezone.
• If we want the container image itself to enforce a system timezone everywhere, the Dockerfile approach is still the stronger mechanism.
• This repo does not currently ship a .devcontainer config, so moving timezone preferences out of the image would mean documenting the expectation for downstream consumers.

Questions to answer

• Do we want the shared dev image to enforce a default system timezone?
• Should timezone preference be treated as a downstream devcontainer concern instead?
• Are there any tools in the shared image that actually require /etc/localtime to be set rather than just TZ?
• If we remove the Dockerfile timezone setup, what documentation should be added for downstream repos?

Done when

• We decide whether timezone belongs in the shared dev image or in downstream devcontainer config.
• The decision is documented in the shared image docs.
• If needed, the Dockerfile and/or downstream guidance are updated to match.