Do not clash with existing Dockerfile

Fixes php-actions/composer#5

php-build.bash

@@ -104,10 +104,10 @@ echo "Building PHP $ACTION_PHP_VERSION with extensions: $ACTION_PHP_EXTENSIONS .
 # it with the unique tag. If the layers are already built, there should be no
 # need to re-build, and the `docker build` step should use the cached layers of
 # what has just been pulled.
-echo "$dockerfile" > Dockerfile
+echo "$dockerfile" > Dockerfile-php-build
 echo "Dockerfile:" >> output.log 2>&1
 echo "$dockerfile" >> output.log 2>&1
-docker build --tag "$docker_tag" --cache-from "$docker_tag" . >> output.log 2>&1
+docker build --tag "$docker_tag" --cache-from "$docker_tag" --file Dockerfile-php-build . >> output.log 2>&1
 # Update the user's repository with the customised docker image, ready for the
 # next Github Actions run.
 docker push "$docker_tag" >> output.log 2>&1