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A dependency can be either added via 'apt-get' (Ubuntu) or 'yum' (Centos) or added by a an 'ExternalProject_Add' function in rdsn/thirdparty/CMakeLitsts.txt.
Adding a third-party currently requires the recompilation of all the other third-parties (usually costs ~4mins on the self-hosted runner, with downloading time of source packages). It needs one Pegasus committer to add a "thirdparty-modified" label to it.
After the PR is merged to master, you should request the owner of "apachepegasus" organization to push the new image of "apachepegasus/ci-env" to dockerhub. (TODO: automates this task)