Fixes using ActiveStorage adapter with libvips as variant processor

[Rails 6.0
introduced](https://github.com/rails/rails/blob/6-0-stable/activestorage/CHANGELOG.md#rails-600beta1-january-18-2019)
the option of using libvips as an alternative to imagemagick as a
backend image processor.

Both processors are handled using the
[image_processing](https://github.com/janko/image_processing) library as
adapter. However, the API for both is not fully compatible. What
it's important for us when generating a variant, the `strip` option
needs to be within the `saver` group to work with both adapters. See
https://gist.github.com/brenogazzola/a4369965a1da426d50f11d080fe2e563#1-move-everything-that-has-to-do-with-compression-to-a-saver-hash
for details. We also need to explicitly coerce size values to integers
for libvips.

We're also adding libvips to the development docker image and allowing to
run tests with that adapter by setting the
`ACTIVE_STORAGE_VARIANT_PROCESSOR` env variable. However, we still
default it to `:mime_magick` (Ruby's wrapper for imagemagick), Rail's
default previous to 7.0. We'll update it once we integrate support for
Rails 7.0.

I also manually tested that metadata is still stripped when using both
adapters.

.dockerdev/Dockerfile

@@ -13,6 +13,7 @@ RUN apt-get update -qq \
     curl \
     git \
     imagemagick \
+    libvips \
     libmariadb-dev \
     sqlite3 \
     libsqlite3-dev \

core/app/models/concerns/spree/active_storage_adapter/attachment.rb

@@ -30,7 +30,9 @@ def variant(style = nil)
         size = style_to_size(style)
         @attachment.variant(
           resize_to_limit: size,
-          strip: true
+          saver: {
+            strip: true
+          }
         ).processed
       end
 
@@ -58,7 +60,7 @@ def metadata
       end
 
       def normalize_styles(styles)
-        styles.transform_values { |v| v.split('x') }
+        styles.transform_values { |v| v.split('x').map(&:to_i) }
       end
 
       def style_to_size(style)

core/lib/spree/testing_support/dummy_app.rb

@@ -96,6 +96,7 @@ class Application < ::Rails::Application
           }
         }
         config.active_storage.service = :test
+        config.active_storage.variant_processor = ENV.fetch('ACTIVE_STORAGE_VARIANT_PROCESSOR', :mini_magick).to_sym
       end
     end
 

docker-compose.yml

@@ -40,6 +40,7 @@ services:
       HISTFILE: "/home/solidus_user/history/bash_history"
       MYSQL_HISTFILE: "/home/solidus_user/history/mysql_history"
       RAILS_ENV: development
+      ACTIVE_STORAGE_VARIANT_PROCESSOR: "mini_magick"
     ports:
       - "${SANDBOX_PORT:-3000}:${SANDBOX_PORT:-3000}"
     volumes: