When expanding build args in a dev container Dockerfile, only the braced
${VAR} form was substituted. The bare $VAR form — which is valid Docker
syntax and common in real-world Dockerfiles — was passed through verbatim.

For example, the Rails dev container images use:

ARG RUBY_VERSION=3.4.4
FROM ghcr.io/rails/devcontainer/images/ruby:$RUBY_VERSION

This left $RUBY_VERSION unexpanded, so the subsequent docker inspect /
build ran against the literal string ruby:$RUBY_VERSION and failed with
an invalid reference format.

This adds expand_dockerfile_var, which substitutes both ${VAR} and bare
$VAR. Bare $KEY is only replaced when it is not immediately followed by
a word character, so expanding $RUBY_VERSION does not partially consume
$RUBY_VERSION2.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed dev container Dockerfiles not expanding build args written in the bare $VAR form