When bootstrapping, check the user's namespace first with their own branch before falling back on matrix-org

https://github.com/matrix-org/sytest/blob/cfbf7430a66d6bcd06b151c7c477ecd2566e1e8b/docker/bootstrap.sh#L32

Edit that to first check `user_repo/sytest@user_branch`, before going to `matrix-org/sytest@user_branch` and then `matrix-org/sytest@develop`

Start at docker/bootstrap.sh around line 32 and inspect the existing repository fallback order. Verify how the bootstrap selects a user's branch, then confirm that it checks user_repo/sytest@user_branch before matrix-org/sytest@user_branch and finally matrix-org/sytest@develop. Done means the selection order is reflected in the script and bootstrap behavior can be verified.

Written by the indexing model from the issue text.