[Dockerfile] Drop minimum security level back to TLS 1.0

Debian increased the minimum security level for OpenSSL from TLS 1.0
to TLS 1.2 [1] which also affects the Debian-based PHP image for Docker.

This change can break some bridges which have to connect to servers with
lower security level. Since all browsers still connect to these servers,
so should RSS-Bridge.

Note that according to [2] Mozilla, Firefox, Microsoft, Google and Apple
plan to increase the minimum security level to TLS 1.2 around March 2020.
At this time RSS-Bridge should follow the browser changes.

This commit updates the Dockerfile to automatically drop the minimum
security level back to TLS 1.0.

Based on the solution provided by @theScrabi in #1318

[1] https://wiki.debian.org/ContinuousIntegration/TriagingTips/openssl-1.1.1
[2] https://salsa.debian.org/ondrej/openssl/blob/553fc8e61f30cd1f7a59dd38c61e1dd4bf58437d/debian/libssl1.1.NEWS

Dockerfile

@@ -6,6 +6,8 @@ RUN mv "$PHP_INI_DIR/php.ini-production" "$PHP_INI_DIR/php.ini" \
 	&& apt-get --yes update && apt-get --yes install libxml2-dev \
 	&& docker-php-ext-install -j$(nproc) simplexml \
 	&& sed -ri -e 's!/var/www/html!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/sites-available/*.conf \
-	&& sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf /etc/apache2/conf-available/*.conf
+	&& sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf /etc/apache2/conf-available/*.conf \
+	&& sed -ri -e 's/(MinProtocol\s*=\s*)TLSv1\.2/\1None/' /etc/ssl/openssl.cnf \
+	&& sed -ri -e 's/(CipherString\s*=\s*DEFAULT)@SECLEVEL=2/\1/' /etc/ssl/openssl.cnf
 
 COPY --chown=www-data:www-data ./ /app/