Update Dockerfile with php 7.1 and added pdftk because it is needed for fillpdf.

build-scripts/drupal7/Dockerfile

@@ -35,22 +35,23 @@ RUN apt-get install -y \
     vim \
     git \
     apache2 \
-    php5.6 \
-    php5.6-apc \
-    php5.6-fpm \
-    php5.6-xml \
-    php5.6-simplexml \
-    php5.6-mbstring \
-    php5.6-cli \
-    php5.6-mysql \
-    php5.6-gd \
-    php5.6-curl \
-    php5.6-ldap \
-    php5.6-mcrypt \
-    php5.6-zip \
+    php7.1 \
+    php7.1-apc \
+    php7.1-fpm \
+    php7.1-xml \
+    php7.1-simplexml \
+    php7.1-mbstring \
+    php7.1-cli \
+    php7.1-mysql \
+    php7.1-gd \
+    php7.1-curl \
+    php7.1-ldap \
+    php7.1-mcrypt \
+    php7.1-zip \
     php-pear \
     libapache2-mod-php5.6 \
     optipng \
+    pdftk \
     jpegoptim \
     imagemagick \
     libapache2-mod-fcgid \