This Docker image is used to build RPM packages in Dockernized envirionment. It's based on CentOS official Docker image.
This image is intended to build mackerel-agent, but maybe you can use this image to build another packages.
Use docker run ... instead of rpmbuild command.
Following example will work as rpmbuild -bb ./SPEC.spec and the artifact will be placed to ./rpmbuild/RPMS/.
$ docker run --rm -v "$(PWD)":/workspace -v "$(PWD)/rpmbuild":/rpmbuild astj:docker-mackerel-rpm-builder -bb SPEC.spec
c5: based oncentos:5c6: based oncentos:6c7: based oncentos:7