Ignore var directory and re-create it in Dockerfile

.dockerignore

@@ -6,3 +6,4 @@ config/console-local.php
 runtime/
 web/assets/
 vendor/
+var/

Dockerfile

@@ -1,7 +1,7 @@
-# This is the Dockerfile to build the production image.
-# It's not used for local development.
-
-# Set this to the version of your base image
+# Application image
+#
+# This image adds the application source to the bas image
+#
 FROM yii2-base-myapp:1.0
 
 # Copy apache and PHP configuration for production into the image
@@ -11,8 +11,6 @@ COPY ./config/php/productive.ini /usr/local/etc/php/conf.d/productive.ini
 # Copy the app code into the image
 COPY . /var/www/html
 
-# The following directories are .dockerignored to not pollute the docker images
-# with local logs and published assets from development. So we need to create
-# empty dirs and set right permissions inside the container.
-RUN mkdir runtime web/assets \
-    && chown www-data:www-data runtime web/assets
+# Create required directories listed in .dockerignore
+RUN mkdir -p runtime web/assets var/session \
+    && chown www-data:www-data runtime web/assets var/sessions