Packages for a release candidate are released as individual files and not as a yum or apt repository. Here's how to install them.
Download the deb file for the rethinkdb package matching your distribution. Do not download the _dbg package. For example:
wget https://thanos.atnnn.com/downloads/rc/2.3.6/2/rethinkdb_2.3.6+0RC2~0jessie_i386.deb
Install it using dpkg -i, for example:
dpkg -i rethinkdb_2.3.6+0RC2~0jessie_i386.deb
Then install the missing dependencies:
apt-get -f install
Download the .dmg file as you would for a normal RethinkDB release. Open the dmg and then run the pkg file. Follow the installation wizard.
Download the .zip file. It contains the standalone RethinkDB.exe executable as-is, with no installer.
Add the RethinkDB packaging key to your keyring:
$ gpg --search-keys CF9BDC6F03D1F02B
gpg: data source: http://74.50.54.68:11371
(1) RethinkDB Packaging <[email protected]>
4096 bit RSA key 0742918E5C8DA04A, created: 2017-07-11, expires: 2019-07-11
Download and verify the signed hash list. For example
$ wget https://thanos.atnnn.com/downloads/rc/2.3.6/2/rethinkdb-v2.3.6-0RC2.sha512sum.sig
$ gpg --decrypt rethinkdb-v2.3.6-0RC2.sha512sum.sig
[...]
f51dd77553c55e5d6ebdd896657a4af80a74c6bdf1895a98ef265e4adfa1b8d3543a9b824826ada6210082bb071abcb33366f0930e67daf9267957f93c377a8e rethinkdb_2.3.6+0RC2~0jessie_amd64.deb
[...]
gpg: Good signature from "RethinkDB Packaging <[email protected]>" [unknown]
Primary key fingerprint: 3B87 619D F812 A63A 8C10 05C3 0742 918E 5C8D A04A
Subkey fingerprint: 4680 C4C9 7EE8 EFAC F82B A780 CF9B DC6F 03D1 F02B
Compare with the hash of the downloaded file. For example
$ sha512sum rethinkdb_2.3.6+0RC2~0jessie_amd64.deb
f51dd77553c55e5d6ebdd896657a4af80a74c6bdf1895a98ef265e4adfa1b8d3543a9b824826ada6210082bb071abcb33366f0930e67daf9267957f93c377a8e rethinkdb_2.3.6+0RC2~0jessie_amd64.deb