The opm package repository appears to be down.
Step 28/31 : RUN opm get 3scale/lua-resty-url
---> Running in d163fe3bf479
* Fetching 3scale/lua-resty-url
ERROR: unexpected server response status code for URL "https://opm.openresty.org/api/pkg/fetch?account=3scale&name=lua-resty-url&op=&version=": 504