Are you for real??
If you have zero case, and you lock down the country or city. that's containment, if you lock down a city with case in it, that's CONFINEMENT, not containment.
You confine the virus in a limited place. Say if your household have it, you lock down that household, that does NOT mean the virus is going away, you are just basically confine the spreading within that household, it would work if R0 is low because that is where its ended after you infected the entire household, but with R0 that high such as Omicron, you are basically seeing a lot more than just that household had them, and you are increasing your chance of cross contamination with other when you have to eventually deal with that household (I assume lock them up and throw out the key until they are non-infecticious is not the Chinese Strategy here), which mean anyone having to have contact with those household are risking of their infection, all the way from government employee to medical personnel to even food delivery people, and with the city being lock down, you can't really control the virus but let them spread in parallel. Which create a breed ground within those household.
On the other hand, if a city is in lockdown, you also limited the health care provided to the citizens because you simply limiting their movement.
So the formula goes, You have a lot of simultaneous infection, a high chance of cross infection, that were giving breeding space within that household, and then limited the health care, it's a recipe for that virus to mutate. Because you keep infecting people that are being limited by the lockdown.