Allows specifying another user to have zsh installed for within environment using -u argument

[balancedscorpion]

This change allows running zsh in a docker container for a non-root user

This addresses the issue described in #27

Primary changes:

1. User Specification:

Added a new option -u to specify the target user.

u)  TARGET_USER=$OPTARG
    ;;

This defaults to the user running the script (usually root) by introducing TARGET_USER variable, defaulting to the current user:

TARGET_USER=$(whoami)

2. Home Directory Determination

Replaced hardcoded home directory with dynamic determination, which changes depending on the target user.

TARGET_HOME=$(getent passwd "$TARGET_USER" | cut -d: -f6)

Testing

I have tested this works for both no argument (default) and a user argument.

Here's the changes required in docker to the script that I used for testing:

Default (no user argument):

No changes are required to the script. But I used my fork to test it:

RUN sh -c "$(curl -L https://raw.githubusercontent.com/balancedscorpion/zsh-in-docker/new_user/zsh-in-docker.sh)" -- \

Specified user

When changing the user, I've first created a user, then added the -u argument (sample credentials below)

RUN useradd -m dockeruser && echo "dockeruser:dockerpassword" | chpasswd && adduser dockeruser sudo

RUN sh -c "$(curl -L https://raw.githubusercontent.com/balancedscorpion/zsh-in-docker/new_user/zsh-in-docker.sh)" --
-u dockeruser \

Then to close out the Dockerfile:

USER dockeruser

CMD ["/bin/zsh"]

[balancedscorpion]

PR is now ready for review and if ok with it to be merged.