Hey guys,
have you considered using Codemagic? It's free; supports github, bitbucket and gitlab; has flutter pre-installed and other software you might need for scripting. Codemagic builds your projects with mac minis and some benefits include team support and automatic provisioning for signing iOS apps (you can build iOS apps without a mac). We don't have support for infrastructure as code yet, that's something we're working on, but we also have integration with Testmagic, which is a hockeyapp alternative for distributing your binaries to testers.