Fix FRANKENPHP_CONFIG environment variable typo in FrankenPHP Dockerfile (#702)

The Dockerfile declared FRANKEN_PHP_CONFIG, but the Caddyfile and the
docs reference FRANKENPHP_CONFIG. Rename the ENV so the default matches
the variable Caddy actually reads.

Author: Abdulmajeed-Jamaan

src/variations/frankenphp/Dockerfile

@@ -146,7 +146,7 @@ LABEL org.opencontainers.image.title="serversideup/php (frankenphp)" \
     COMPOSER_HOME=/composer \
     COMPOSER_MAX_PARALLEL_HTTP=24 \
     DISABLE_DEFAULT_CONFIG=false \
-    FRANKEN_PHP_CONFIG="" \
+    FRANKENPHP_CONFIG="" \
     LOG_OUTPUT_LEVEL=info \
     HEALTHCHECK_PATH="/healthcheck" \
     PHP_DATE_TIMEZONE="UTC" \