This replaces the use of the awscli utility. awscli binary is massive,
it added about 200 MB to the docker image size, while the s3 client was
already a dependency so using that is essentially free, as far as binary
size is concerned.
I implemented a simple upload function that tries to keep 10 uploads
going in parallel. I believe that's the default behavior of the "aws s3
sync" command too.