Jenkins: where is the login.keychain file in /Users/*name*/Library/Keychains folder?
You can simply unlock your keychain through shell.In your Jenkins project configuration, add an 'execute a script shell' step :
security unlock-keychain -p PASSWORD ${HOME}/Library/Keychains/login.keychainsecurity set-keychain-settings -t 3600 -l ${HOME}/Library/Keychains/login.keychainNote. The 'set-keychain-settings' leaves the keychain unlocked for a while (3600s), so your build process (xcodebuild in your case) still have access to the keychain when it's signing the IPA.
Note 2. The keychain extension changed with Sierra, it's now .keychain-db indeed. Using directly the security command won't affect you.