Update Dockerfile (#15)

Co-authored-by: Andy Miller <1084697+rhukster@users.noreply.github.com>

Dockerfile

@@ -11,6 +11,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libjpeg62-turbo-dev \
     libpng-dev \
     libyaml-dev \
+    libzip4 \
     libzip-dev \
 	  cron \
 	  vim \
@@ -34,7 +35,7 @@ RUN { \
 	} > /usr/local/etc/php/conf.d/php-recommended.ini
 
 RUN pecl install apcu \
-    && pecl install yaml \
+    && pecl install yaml-2.0.4 \
     && docker-php-ext-enable apcu yaml
 
 # Set user to www-data
@@ -45,7 +46,6 @@ USER www-data
 ENV GRAV_VERSION 1.6.19
 ENV GRAV_SHA1 231e6789e9575adccd6044aa0d0c72b8c2603a96
 
-
 # Install grav
 WORKDIR /var/www
 RUN curl -o grav-admin.zip -SL https://getgrav.org/download/core/grav-admin/${GRAV_VERSION} && \