Docker command failing on ARM
Did you clone the repository recursively?
It seems that bin/getpy3 is actually included in deps/readies as a git submodule (see here).You thus might need to update the submodules. You might want to try to initialize the submodule and retry the build :)
git submodule update --init --recursive
Did you clone https://github.com/RedisTimeSeries/RedisTimeSeries to the directory your dockerfile is in? Step 6 is copying the getpy3 file (and others) into the image, and step 8 (fails) is attempting to run it.
Host side, you're missing $PWD/deps/readies/bin/getpy3.
Their github seems to also have an arm flavored dockerfile: https://github.com/RedisTimeSeries/RedisTimeSeries/blob/master/Dockerfile.arm