Fix envbuilder build failure from unset _REMOTE_USER_HOME during feature installs

[Copilot]

The Build Envbuilder CI job fails while installing the claude-code feature:

cp: cannot stat '/.local/bin/claude': No such file or directory
error: do build: ... exit status 1: no fallback image has been specified

Coder builds workspaces with envbuilder, which injects only _CONTAINER_USER and _REMOTE_USER into feature install.sh runs — never _REMOTE_USER_HOME. The feature's final step expands to cp /.local/bin/claude ... and aborts the build. The devcontainer CLI sets the variable, so only the envbuilder path breaks.

Changes

• .devcontainer/Dockerfile: declare the home-dir variables envbuilder omits, so feature installs can locate the target user's home:

  ARG _REMOTE_USER_HOME=/home/node
  ARG _CONTAINER_USER_HOME=/home/node

Why ARG

• envbuilder appends feature RUN steps after this Dockerfile in the same stage, so the ARGs stay in scope and supply the missing values.
• Unlike ENV, ARGs don't persist into the running container's environment.
• Under the devcontainer CLI the ARG scope resets and the CLI sets these itself — no-op there.

Scope

Benefits any feature whose install.sh reads _REMOTE_USER_HOME/_CONTAINER_USER_HOME, not just claude-code.