Avocados are generally averse to city water, probably due to the chlorine?
If you use a peat-based potting mix, there is probably enough buffer capacity to allow you to water with high pH water for years without issue. For example, I've been using canal water (laced with loads of calcium carbonate) for years on plants in the same peat-based mix, but pH is still well below 7.
But it's worth noting that avocados don't make the best container specimens. They will probably do well initially, but once they max out the container with roots, they will start to decline a little. I haven't done a lot of container culture, but if this is your long term plan, my guess is that you're going to need to root prune periodically.