Update images, fix extrapath (#723)

* fix: typo

* fix: remove debug for git references

it generates a massive amount of logs

* feat: read values from env

allows to test act on GHA when it's not a main repo

* fix: merge extrapath with PATH

* fix(tests): add additional shells for testing

* fix(image): update images

pin node to major version only, current node version: 12.22.1
replace most images with `node:12-buster-slim` to prevent
errors on macOS runner due to DockerHub pull limit
replace ocaml image

Co-authored-by: Casey Lee <cplee@nektos.com>

pkg/runner/testdata/parallel/push.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [build]
     steps:
-      - uses: docker://ubuntu:18.04
+      - uses: docker://node:12-buster-slim
         with:
           args: echo ${GITHUB_REF} | grep nektos/act
       - uses: ./actions/docker-url