Change Travis CI to use a Docker image of Flutter

[ctiedt]

trafficstars

Is your feature request related to a problem? Please describe. Because Travis pulls the Flutter GitHub repository every time, the build takes a long time to run.

Describe the solution you'd like Using a Flutter Docker image should make the build process significantly faster.

[MarcelGarus]

We could also try out GitHub Actions.

[JonasWanke]

We're actually not using a docker image yet but download the release bundle every time