I wrote about building your code on Azure DevOps earlier:
https://daxvisionerp.com/dynamics-365-fo-build-from-release-folder/

The build agent is running a virtual machine. You need to start the VM to be able to start a build. This has some costs most of the time.

Something is happening related to this topic. Soon you will not need to use a virtual machine as a build machine. See this post from Microsoft: https://docs.microsoft.com/en-us/dynamics365-release-plan/2019wave2/finance-operations-crossapp-capabilities/build-automation-hosted-azure-devops

Joris de Gruyter from Microsoft also mentioned this in one of his presentations that I saw:
“A new build experience is coming, without VM
Simple and standard .NET
No VM required
Free on Azure DevOps hosted agents, with minutes per month”

I will let you know when I know more, or feel free to leave a comment if you have more information about this.