Migrate CI builds to GitHub Actions

[ignasi35]

Projects relying on docker to run a step of their CI build should move to GH Actions as soon as possible because of dockerhub rate-limiting policy affecting Travis.

• https://github.com/akka/akka-management (in progress release to sonatype using GH Actions akka-management#889)
• https://github.com/akka/akka-persistence-jdbc
• https://github.com/akka/akka-persistence-cassandra
• https://github.com/akka/akka-persistence-spanner
• https://github.com/akka/akka-persistence-couchbase
• https://github.com/akka/alpakka
• https://github.com/akka/alpakka-kafka
• https://github.com/akka/akka-projection

Like in #149, it is OK if we only migrate the jobs of the complete CI build that need docker. It is not the goal of this issue to do a full migration from Travis to GitHub actions. The goal is to make builds less flaky.

[ennru]

We're good here.