Add support for running ugit inside docker

[Bhupesh-V]

Is your feature request related to a problem? Please describe.

We are planning to ship a docker image for ugit, which everyone would be able to pull from dockerhub registry. We need to make few adjustments in ugit for that.

Describe the solution you'd like

To aid this, we will introduce a new boolean ENV variable UGIT_RUNNING_IN_DOCKER. Use this new variabel to perform following things:

• Don't open the guide when user invokes the script with --guide flag. Instead, just display the URL of the guide.
• Don't update the script when user invokes the script using --update flag, just print what version of script a user is using, and tell them to pull the latest image.

Other items that are related to this:

• Remove usage of HEREDOC in favor of printf while printing --help text #67
• remove usage of basename on --help argument #66

Describe alternatives you've considered

No response