bug: devcontainer.Compile references global variable

[johnstcn]

If you try to compile a Dockerfile from a devcontainer.json as non-root, you will get this error:

Error: compile devcontainer.json: open dockerfile: mkdir /.envbuilder: permission denied

(Alternatively, you may also get a "read-only filesystem" error.)

We should instead write the Dockerfile into the build context and not just use MagicDir.