fix: substitute devcontainer.json build args into Dockerfile ARG variables

[kacpersaw]

Fixes#455

Problem

When devcontainer.json specifies build args (e.g. "VARIANT": "3.11-bookworm") and the Dockerfile uses ARG VARIANT + FROM ...${VARIANT}, envbuilder fails because ImageFromDockerfile and UserFromDockerfile don't accept external build args — they only use ARG defaults from the Dockerfile itself.

Changes

• ImageFromDockerfile — Added buildArgs []string parameter. Build args now override ARG defaults and supply values for ARGs without defaults.
• UserFromDockerfile — Added buildArgs []string parameter. Collects ARG values with build arg overrides and substitutes them into stage.BaseName before parsing image refs.
• Updated all call sites to pass build args through.
• Added tests for both override and no-default cases.

[johnstcn]

One point I'd like to raise here: if you try to build this Dockerfile it will probably error with something like

ERROR: failed to build: failed to solve: failed to parse stage name "python:1-": invalid reference format

whereas a different Dockerfile would probably not fail e.g.

FROM alpine:3.18
ARG VARIANT
RUN echo "foo-${VARIANT}" > /tmp/test.txt && cat /tmp/test.txt
EOF

Docker also seems to have some kind of "special-case" warning about this e.g.

�[0m - InvalidDefaultArgInFrom: Default value for ARG alpine:${VARIANT} results in empty or invalid base image name (line 2)

[twsl]

feel free to check #463 and close it once this is merged