Added configurable user for shell commands

[fkrzski]

Reopened PR #779 (I don't see Reopen pull request button`

Problem

Laravel Sail defaults executing commands as sail user in a container, which causes problems in some environments where:

• Specific user is required
• There is a need to integrate with external system with specific user
• Development teams have set standards for users in containers (My case)
• Users have problems with file permissions

Solution

I introduce APP_USER env variable, which:

• Defaults to sail to preserve backwards compatibility
• Aloows developers to specify their own user in the .env. file
• Is used in all commands where -u sail was previously set

Benefits

• Developers can customize the container without modifying the source code
• Easier integration with existing file systems and databases
• The same user can be used in development, testing and production
• Eliminates problems with file permissions during development

Backward compatibility

• Default value "sail" provides identical behavior as before
• Existing installations that do not define the new variable are not affected

Impact

Low