Add libssl-dev to support mongodb ssl/tls connections. (#67)

Closes #67
diff --git a/core/php7.3Action/Dockerfile b/core/php7.3Action/Dockerfile
index 308dc64..a7663e5 100644
--- a/core/php7.3Action/Dockerfile
+++ b/core/php7.3Action/Dockerfile
@@ -34,6 +34,7 @@
       libicu57 \
       libjpeg-dev \
       libpng-dev \
+      libssl-dev \
       libxml2-dev \
       libzip-dev \
       postgresql-server-dev-9.6 \