Apply thread allocations patch to Ruby 2.7

This makes us to compile Ruby 2.7 manually
and apply relevant patch.

Dockerfile.custom

@@ -13,6 +13,12 @@ RUN /scripts/install-essentials
 
 ENV PATH $PATH:/usr/local/go/bin
 
+# Ruby
+ARG RUBY_VERSION
+ARG RUBY_DOWNLOAD_SHA256
+
+RUN if [ -n "$RUBY_VERSION" ]; then /scripts/install-ruby $RUBY_VERSION $RUBY_DOWNLOAD_SHA256 && ruby --version; fi
+
 # Git
 ARG GIT_VERSION
 ARG GIT_DOWNLOAD_URL