Inline buildx global build and target platform envvars when resolving base image and user

[trxcllnt]

This PR ensures the buildkit global platform ARGs are respected in findBaseImage() and findUserStatement() functions.

These variables are valid in FROM lines, e.g.:

FROM ubuntu:latest as base-amd64
ENV MY_ARCH=amd64

FROM ubuntu:latest as base-arm64
ENV MY_ARCH=arm64

FROM base-${TARGETARCH}

ARG TARGETARCH

Without this change, the devcontainers CLI attempts to pull an image named base- rather than continue to walk the FROM tree to find ubuntu:latest.

Fixes #275