Adding the missing gnupg2 package

Dockerfile

@@ -1,9 +1,9 @@
 FROM php:7
 MAINTAINER Carlos A. Gomes <carlos.algms@gmail.com>
 
-RUN \
-  apt-get update \
+RUN apt-get update \
   && apt-get install --no-install-recommends --no-install-suggests -y \
+      gnupg2 \
       apt-transport-https \
   && curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - \
   && echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list \
@@ -16,8 +16,6 @@ RUN \
       rsync \
       git \
       yarn \
-
-
   # Slim down image
   && apt-get clean \
   && rm -rf \
@@ -26,7 +24,6 @@ RUN \
       /var/tmp/* \
       /usr/share/man/?? \
       /usr/share/man/??_* \
-
   # Install composer
   && curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer \
   # Show versions