refactor(entrypoint) improve script clarity

[joshtrichards]

Improved entrypoint script clarity:

• General code reformatting
• Refactors to keep most lines <=90 characters
• Updates existing comments for clarity/consistency/usefulness
• Adds new, clarifying comments
• Adds/enhances descriptions for all utility functions
• Adds reminder language re: ash/dash script compatibility
• Minor adjustments to some error/general output (cosmetic only)
• Notes some future "TODO" (possible) items
• Documents the current reasoning for deploying code onto persistent volume

No logic changes per se but:

• Moves rsync base config/callilng into a dedicated function
• Fixes non-POSIX variable compliance
• Adjusts how the PHP Redis session config file is built for clarity purposes (content does not change)