Add '-o' to groupadd and useradd to fix error

[hachi8833]

• macOS: 14.4.1 Sonoma
• Docker Desktop for Mac: 4.28.0 (139021)
• cargo 1.74.1 (ecb9851af 2023-10-18)

As far as I checked this tool by runningcargo clean;cargo build;cargo install --path . and then path/to/rails-new app_name --main, the following error occurs:

 => ERROR [2/3] RUN groupadd -g 20 app && useradd -u 501 -g app -m app                      0.1s
------
 > [2/3] RUN groupadd -g 20 app && useradd -u 501 -g app -m app:
0.132 groupadd: GID '20' already exists
------
Dockerfile:5
--------------------
   3 |     ARG USER_ID=1000
   4 |     ARG GROUP_ID=1000
   5 | >>> RUN groupadd -g $GROUP_ID app && useradd -u $USER_ID -g app -m app
   6 |     USER app
   7 |     ARG RAILS_VERSION
--------------------
ERROR: failed to solve: process "/bin/sh -c groupadd -g $GROUP_ID app && useradd -u $USER_ID -g app -m app" did not complete successfully: exit code: 4

View build details: docker-desktop://dashboard/build/desktop-linux/desktop-linux/ise7q36c2kuyza1pnuw26km5y
thread 'main' panicked at src/main.rs:41:5:
assertion failed: status.success()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I added -o options to groupadd and useradd to fix the error.

[rafaelfranca]

Thank you for the pull request. I don't think we should override a group id or user id already existent in the system.

In the case of MacOS, it is probably a better option to not create a new group.