ci: remove unused env var

Signed-off-by: rare-magma <rare-magma@posteo.eu>

Dockerfile.alpine

@@ -43,7 +43,6 @@ ENV TELEMETRY=false
 ENV ENABLE_ID_OBFUSCATION=false
 ENV REDACT_IP_ADDRESSES=false
 ENV WEBPORT=80
-ENV APACHE_DOCUMENT_ROOT=/var/www/html
 
 # https://httpd.apache.org/docs/2.4/stopping.html#gracefulstop
 STOPSIGNAL SIGWINCH