PM for GitHub Actions here. I wouldn't suggest that you build an action to run a build within a container. We offer container support for workflow runs which should be suitable for this. (Either by specifying the `jobs.<job-id>.container` or by using `docker run ...`)
Actions themselves are meant to support the workflow and distill a set of complex steps into something that can be done in a single workflow command. (For example, download and set up a new version of a Java JDK.) They're meant to be used as steps in many peoples workflows, instead of executing a single workflow.
The actions themselves can be built as either a container, which is self-contained, or as JavaScript. I often recommend the latter, since that will work cross platform (containers only work for people who are using workflows running on Linux).
Actions themselves are meant to support the workflow and distill a set of complex steps into something that can be done in a single workflow command. (For example, download and set up a new version of a Java JDK.) They're meant to be used as steps in many peoples workflows, instead of executing a single workflow.
The actions themselves can be built as either a container, which is self-contained, or as JavaScript. I often recommend the latter, since that will work cross platform (containers only work for people who are using workflows running on Linux).