Try a multi-stage build. Fetch your build dependencies in a "modern" environment, like "FROM centos:7 AS build", then drop a "FROM centos:5" in the middle, and

Try a multi-stage build. Fetch your build dependencies in a "modern" environment, like "FROM centos:7 AS build", then drop a "FROM centos:5" in the middle, and "COPY --from=build" the stuff you need.