feat(redis): Eliminate runtime-generated redis-session.ini

[joshtrichards]

Fixes #763

• avoids stale redis-session.ini
• reduces mutable filesystem state
• makes PHP config more declearative
• aligns Redis PHP session config with the existing env-substitution pattner already used for other PHP settings like PHP_MEMORY_LIMIT
• eliminates incompatibility of Redis session handler configuration with "rootless" scenarios (Introduced redis session handler does not work with non-root images #763)

Requirements:

• Existing container-level REDIS_* image configuration variables remain as they exist today
• Redis is enabled only when configured

For standard environments (i.e. those that already work today with the existing implementation), the result should be:

• No change

For environments running containers w/o root:

• They should start working (i.e. using the specified REDIS_HOST as the PHP session handler... instead of generating an error about being unable to write to the ini file and falling back on file-based session storage)

This is a prototype in its current state.

Key things to confirm:

• What happens when the env vars are unset?

P.S. If the empty values being always passed through is problematic.... This tests the full “drop redis-session.ini entirely” idea. If this runs into challenges, we could generate a separate ini file (only when Redis is configured) in the entrypoint, but still source values from env. That would test the env-based composition separately from the fallback problem.