I found this little bit of info on imdb
Shooting the five minute Dunkirk beach scene was arguably the toughest portion of shooting. The shooting schedule dictated that the scene must be completed in two days, because the crew has limited time with the 1,000 extras. However the location scouts report indicated the lighting quality at the beach was not good enough until the afternoon of the second day. This forced director Joe Wright to change his shooting strategy into shooting with one camera. The scene was rehearsed on the first day and on the morning of the second day. The scene required five takes and the third take was used in the film. On shooting, Steadicam operator Peter Robertson shot the scene by riding on a small tracking vehicle, walking off to a bandstand after rounding a boat, moved to a ramp, stepped onto a rickshaw, finally dismounting and moving past the pier into a bar.