stunnel 5.50 (Container beta3)

• Version-locks Alpine Linux to 3.8 for CI stability
• Makes cipher list configurable at runtime, via the new STUNNEL_CIPHERS environment variable (see the README).
• Switches from the grade "A+" cipher list to the grade "A" cipher list, to accommodate Azure's implementation of TLS for Redis does not support any of the ciphers in the A+ list. In practice it wants ECDHE-RSA-AES256-SHA384.
• Eliminates on-disk logging, in favor of writing to standard out and standard error.