add alpine linux compatibility

[ajbt200128]

Hey y'all, this adds support for alpine when running this action.

[smorimoto]

Adding features that are too specialized for the various distributions used by self-hosted runners is out-of-scope for this action. The installation of system packages can be accomplished by running in the previous step of this action.

[ajbt200128]

@smorimoto do you mind if I create an issue then? This action will always run apt to install packages if the OS is linux, making it unusable on self hosted runners

[ajbt200128]

(I also don't mind fixing this myself)

[smorimoto]

https://github.com/ocaml/setup-ocaml/blob/master/EXAMPLES.md#using-with-containers

[ajbt200128]

this example doesn't work if the container isn't ubuntu/debian, as when initializeOpam is called, if the container is linux, it will always try to use apt

[smorimoto]

@ajbt200128 The fix is ​​included in the latest release.

[ajbt200128]

thanks!