Regression: Removal of USER vscode from Ruby Dockerfile is breaking other projects

[jchilders]

This was discovered as a result of rails/rails#53581.

Currently, devcontainers are failing for both rails and rails-contributors. The root cause for each appears to be the changes introduced in #51. Reverting those changes allows the devcontainers to be built without error.

See this PR comment for the steps to reproduce the error. To summarize, though: reverting #51 fixes the problems in building the devcontainers for both projects.

The question is whether #51 should be reverted, or whether dependent projects should be updated.