Fix pulling Dart in the nginx Dockerfile

[mattlorimor]

Problem

As referenced in #1206, the Docker build is currently broken due to not being able to pull Dart 1.24.* from the package list (it seems to have been removed).

Fix

Pull 1.24.3 of the Dart SDK directly so that the get and build still work.

[mattlorimor]

@mikegrima - Per your request ^^^

[mattlorimor]

Build fails, but it doesn't appear to be because of this change.

[mikegrima]

hmm look like an issue with moto.

[mattlorimor]

@mikegrima

Two of the tests seem to be broken:

• RDSSubnetGroupWatcherTestCase
• LambdaFunctionWatcherTestCase

For RDSSubnetGroupWatcherTestCase, I think

subnet = vpc_conn.create_subnet(vpc.id, "10.1.0.0/24")

just needs to change to

subnet = vpc_conn.create_subnet(vpc.id, "10.0.0.0/24")

I can't figure out why len(item_list) is suddenly returning 2 in LambdaFunctionWatcherTestCase, though.