Ensure fast flush of HTTP headers and HTML head

The fast flush optimisation done in
FreshRSS#1133 does not seem to work
anymore (need to check if it is related to a PHP version).
Work-around when PHP flush() is not working
Can be tested by adding a `sleep(5);` after:

https://github.com/FreshRSS/FreshRSS/blob/ee902ee7c4370421802768c3105ba269a4f97b16/app/layout/layout.phtml#L27
Follow-up of the performance checks of
FreshRSS#2040

Docker/Dockerfile

@@ -14,6 +14,7 @@ WORKDIR ${FRESHRSS_ROOT}
 
 COPY . ${FRESHRSS_ROOT}
 COPY ./Docker/*.Apache.conf /etc/apache2/conf.d/
+COPY ./Docker/*.php.ini /etc/php7/conf.d/
 
 RUN sed -r -i "/^[ ]*(CustomLog|ErrorLog|Listen) /s/^/#/" /etc/apache2/httpd.conf && \
 	echo "17,37 * * * * php ${FRESHRSS_ROOT}/app/actualize_script.php 2>&1 | tee /tmp/FreshRSS.log" >> \

Docker/FreshRSS.php.ini

@@ -0,0 +1,2 @@
+; Keep below ~2600 to ensure fast headers when flush() is not working
+output_buffering = 2500