FEATURE: SKIP_DB_AND_REDIS env var (#7756)

Sometimes we would like to create a base image without any DB access, this
assists in creating custom base images with custom plugins that already
includes `public/assets`

Following this change set you can run:

```
SPROCKETS_CONCURRENT=1 DONT_PRECOMPILE_CSS=1 SKIP_DB_AND_REDIS=1 RAILS_ENV=production bin/rake assets:precompile
```

Then it is straight forward to create a base image without needing a DB or
Redis.

lib/discourse.rb

@@ -245,7 +245,13 @@ module Discourse
   end
 
   def self.cache
-    @cache ||= Cache.new
+    @cache ||= begin
+      if GlobalSetting.skip_redis?
+        ActiveSupport::Cache::MemoryStore.new
+      else
+        Cache.new
+      end
+    end
   end
 
   # Get the current base URL for the current site