Yeh. I haven't yet figured out how to get zram to apply transparently to containers though, anything in another memory cgroup will never get compressed unless swap is explicitly exposed to it.
The backing disk or file will only be written to if cache eviction on the basis of LRU comes into play, which is fine because that's probably worth the write hit. The likelihood of thrashing, the biggest complaint about disk based swap, is far reduced.
zram based swap isn't free. Its efficiency depends on the compression ratio (and cost).
It'd be cool if Zram could apply to the RAM itself (like macOS) rather than needing a fake swap device.