You can approach this case with multiple ways.
For example you can set an auto-shutdown for 20:00 pm and auto-start for 8:00 am.
Use Terraform to create your environment and, after deployment/testings, you can easily destroy it, saving your costs.
And many other ways.
I commented on paths that you can follow, so you should choose one that brings you ...
You've missed the part of the 1st quote about multi-stage builds and other methods which avoid exactly the problem mentioned in the 2nd quote: the resulting image would consist of it's base image and artifacts from previous stages while being run in the same build step of a pipeline.
For less trafficked parts of your web service, synthetic transactions are a good option.
There are some trade-offs to be made when choosing either using an API to or using a full browser style check.
API checks require changing the application being tested. I'd try and go this route first, but it is dependant on the functionality you need to test. These can ...