B737 flight crew reported loss of hydraulic fluid during taxi for takeoff due to failure of hydraulic line. Aircraft was towed back to the gate.

Narrative

We pushed back and taxied out normally; went to park on the taxiway to await ATC release to ZZZ. Shut down the engines; leaving both electric HYD pumps on. After about 10 minutes we received a HYD master caution light; and a low pressure light off our electric A pump. Once we flipped to the HYD status screen; system A was showing a quantity of 0 and PSI fluctuating in the 100s. Both systems showed quantities above 90 percent and PSI around 3000 during the before push checklist. Captain consulted Maintenance Control and we chose to return to the gate; electing to use a tow crew over switching to alternate nose wheel steering with the loss of system A. We also shut down both pumps to system A. Tow crew arrived and reported a pool of hydraulic fluid near the left main gear. Upon reaching the gate and deplaning Maintenance discovered that a line connecting to the gear actuator had failed; causing System A's fluid to be dumped on the taxiway.Part was flown in from ZZZ1; replaced; and the original crew completed an uneventful flight to ZZZ.