EMB-175 Captain reported the aircraft had a abnormal start; and rolled uncontrolled while parking at the gate due to their failure to set the parking brake.

Narrative

When departing initially out of ZZZ we had an engine abnormal start on engine #2 after the attempted start resulted in no n2 rotation or positive oil pressure. This caused us to pull back into the gate; deplane; and have maintenance come on to fix the affected engine. They were able to fix the problem which we were told was an issue with the starter and after a few tests we were put back into service. Since becoming a Captain this was the first time I have had to deal with any kind of engine issue and so admittedly I was a little bit nervous about keeping the same plane and engine after all this. But again the test checks were all showing successful and everything was running normally so I accepted the plane; we boarded the passengers again and went on our way. We took off without incident and flew and landed in ZZZZ without incident and then we pulled into the gate.At this time the ramper marshaled us in and as he gave us the stop 'x' with his wands. I made sure the thrust was idle and then shut down the engines. At this moment I didn't realize it yet but I should have set the parking brake first as we all normally do right when we get that stop signal but I was still distracted by the earlier event with the engine and wanted to monitor the shutdown more closely to make sure there were no abnormal indications or anything and that distracted me just enough to get me off of my normal shutdown flow this time and miss that step of setting the parking brake. I looked up after doing the remainder of the flow to wait for the chocks in signal from the ramper who marshaled us in but noticed we were slowly moving backwards so I stepped on the brakes again and noticed the parking brake wasn't set so I set it and that was the end of it. The gate agent driving the jet bridge asked us if we noticed we were moving backwards a little and I told them yes and sorry about that; the gate agent mentioned that this had happened previously at this same gate so they were trying to figure out what might have caused that and I told them I just didn't have the brake set yet. We then made sure nobody was injured or any equipment damaged. there was none reported so we deplaned and then boarded the next flight to head back to ZZZ without any further incident.Mainly just distraction and thoughts being occupied by this previous engine abnormal start. The fact that we were now almost 3 hours behind schedule also played a factor because now we were all in a bit of a hurry up and try to get caught up mindset. Obviously this is a mistake that shouldn't really happen because 99.9% of the time you get to the gate and the first thing you do is set the brake when you park but again when your mind gets fixated on other things even these simple and routine tasks can unfortunately slip the mind sometimes.I don't have any grand ideas of how to avoid recurrence of this event other than possibly marking on our company charts that the pavement isn't flat on this particular gate because the gate agent did mention that a rollback has happened at this gate before so maybe it could be an ongoing issue and extra crew awareness could help solve this. But ultimately by properly executing the shutdown flow would have solved this issue because then the parking brake would have been set and no rollback would have occurred.