ci: Fix push-docs: Add github.com host key to known hosts

tests/travis/push-documentation

@@ -4,6 +4,8 @@
 #
 # SPDX-License-Identifier: CC0-1.0
 
+set -e
+
 # Add ssh push key
 eval "$(ssh-agent -s)"
 echo $PUSH_KEY_REPO_DOC_QXMPP_ORG | base64 -d | unxz | ssh-add -
@@ -35,7 +37,14 @@ echo '-------------------------------------------'
 echo 'Cloning doc.qxmpp.org repository'
 echo '-------------------------------------------'
 
-git clone ssh://[email protected]/qxmpp-project/doc.qxmpp.org
+# add github.com to known hosts
+mkdir -p ~/.ssh
+touch ~/.ssh/known_hosts
+ssh-keyscan -t ed25519 github.com >> ~/.ssh/known_hosts
+ssh-keyscan -t rsa github.com >> ~/.ssh/known_hosts
+
+ssh -Tv [email protected] -o StrictHostKeyChecking=no
+GIT_SSH_COMMAND="ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no" git clone ssh://[email protected]/qxmpp-project/doc.qxmpp.org
 
 echo '-------------------------------------------'
 echo 'Commiting and pushing documentation'