Fix permission issues with docker mount, and update related documentation

[pcmxgti]

Description

Update Dockerfile and documentation on how to run Tokendito from a Docker container

Motivation and Context

Previous versions of the documentation did not explain clearly how to have read/write mounts on Linux so that Tokendito could update the AWS credentials files

How Has This Been Tested?

Locally on Linux (Ubuntu) and MacOS

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes