Configure static to work with dartsass in govuk-docker

[AshGDS]

What

• Adds code to support dartsass compilation in govuk-docker, based off of Enable automatic recompiling of stylesheets whitehall#8812
• One thing to mention is docker didn't like the execution of bin/dev so I had to run chmod +x bin/dev on my machine. Not sure if this will be an issue for others?

Why

• static has not been updating its SASS/CSS properly when govuk_publishing_components is updated. This is due to the migration to dartsass.

How to test

• Checkout this branch
• Change your govuk-docker/static/docker-compose.yml to use command: bin/dev instead of command: bin/rails s --restart (or just checkout this branch of govuk-docker: Use dartsass for static css compilation govuk-docker#721)
• Change static to use your local govuk_publishing_components then make static / run static
• When you change SASS in govuk_publishing_components, it should update instantly in static