CI: fix build-docker-args task

This task doesn't have a cess to bosh-linux-stemcell-builder so it
needs to read `.ruby-version` from it's own resource.
Also add `.ruby-version` to the `bosh-stemcells-ci` resources's paths.

Author: aramprice

ci/pipelines/builder.yml

@@ -1060,6 +1060,7 @@ resources:
     branch: (@= data.values.stemcell_details.branch @)
     paths:
       - ci
+      - .ruby-version
     uri: https://github.com/cloudfoundry/bosh-linux-stemcell-builder
 
 - name: bats

ci/tasks/build-docker-args.sh

@@ -28,7 +28,7 @@ yq_cli_url="$(curl -s https://api.github.com/repos/mikefarah/yq/releases/latest
 ruby_install_url="$(curl -s https://api.github.com/repos/postmodern/ruby-install/releases/latest \
                     | jq -r '.assets[] | select(.name | endswith ("tar.gz")) | .browser_download_url')"
 
-ruby_version="$(cat "${REPO_PARENT}/bosh-linux-stemcell-builder/.ruby-version")"
+ruby_version="$(cat "${REPO_ROOT}/.ruby-version")"
 gem_home="/usr/local/bundle"
 
 cat << EOF > "${REPO_PARENT}/docker-build-args/docker-build-args.json"