Use newer base PHP image

Remove memcached extension installation, because it's done in the parent
image now

Dockerfile

@@ -1,28 +1,3 @@
-FROM git.picklesniffers.com/rock.ru/php:5.3-6
+FROM git.picklesniffers.com/rock.ru/php:5.3-7
 
 COPY src /var/www/rock/forum
-
-RUN set -eux; \
-    apt-get update; \
-    apt-get install -y --no-install-recommends \
-      libmemcached-dev \
-      libz-dev \
-    ; \
-    apt-get clean; \
-    rm -rf /var/lib/apt/lists/*
-
-RUN set -eux; \
-    cd /tmp; \
-    curl -JLO https://github.com/php-memcached-dev/php-memcached/archive/refs/tags/2.2.0.tar.gz; \
-    tar xpf php-memcached-2.2.0.tar.gz; \
-    cd php-memcached-2.2.0; \
-    phpize; \
-    ./configure; \
-    make install; \
-    cd /tmp; \
-    rm -rf php-memcached-2.2.0.tar.gz php-memcached-2.2.0
-
-RUN set -eux; \
-  { \
-    echo 'extension=memcached.so'; \
-  } | tee /usr/local/etc/php/conf.d/memcached.ini;