Dockerfile Nonexistant Branch Checkout in WALLY

**Additional context**
/backend/dockerfile.dev has a git checkout command that causes the build to fail because the branch it references does not exist.

**Definition of Done**
- [ ] Remove deprecated git checkout command

Open /backend/dockerfile.dev and inspect the git checkout command identified in the issue. Remove the deprecated checkout command, then run the Docker build to confirm it no longer fails because of the nonexistent branch.

Written by the indexing model from the issue text.