Set up clamav in Dockerfile
Set up clamav in Dockerfile

This installs clamav, updates the definition files and creates a symlink
to govuk_clamscan

It's not ideal really:
- It takes ages to set up - and currently it's only running in a test
  environment;
- freshclam errors due to lack of clamd running (more specifically a
  lack of clamd config file)

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager