Add xdebug to php-unit-test images

xdebug is required for code coverage reports and is not included in the default php Alpine images.

$PHPIZE_DEPS is defined by the PHP Alpine image maintainers and documented as available for use in Dockerfiles. It will contain a list of packages needed to build PHP extensions.

php-unit-test/5.6/Dockerfile

@@ -12,7 +12,10 @@ LABEL org.label-schema.build-date=$BUILD_DATE \
       org.label-schema.schema-version="1.0" \
       maintainer="Alan Nelson <alan.nelson@nebraska.edu>"
 
-RUN apk add --no-cache bash nodejs nodejs-npm
+RUN apk add --no-cache bash nodejs nodejs-npm $PHPIZE_DEPS \
+    && pecl install xdebug-2.5.0 \
+    && docker-php-ext-enable xdebug \
+    && apk del --no-cache $PHPIZE_DEPS
 RUN curl -o /usr/local/bin/composer https://getcomposer.org/composer.phar \
   && chmod 755 /usr/local/bin/composer