CI Docker diffs - use env.MATCHED_FILES

With update to GitHub Actions technote-space/get-diff-action, we need to use env.MATCHED_FILES instead of env.GIT_DIFF to see if any of the files listed under FILES have been changed. env.GIT_DIFF basically always returns true, so currently the Docker image always builds.

Docs: https://github.com/marketplace/actions/get-diff-action#examples-of-env

Original PR: nf-core#774

nf_core/pipeline-template/{{cookiecutter.name_noslash}}/.github/workflows/ci.yml

@@ -33,11 +33,11 @@ jobs:
             environment.yml
 
       - name: Build new docker image
-        if: env.GIT_DIFF
+        if: env.MATCHED_FILES
         run: docker build --no-cache . -t {{ cookiecutter.name_docker }}:dev
 
       - name: Pull docker image
-        if: {% raw %}${{ !env.GIT_DIFF }}{% endraw %}
+        if: {% raw %}${{ !env.MATCHED_FILES }}{% endraw %}
         run: |
           docker pull {{ cookiecutter.name_docker }}:dev
           docker tag {{ cookiecutter.name_docker }}:dev {{ cookiecutter.name_docker }}:dev