To make it less confusing what version of bitcoind is actually
installed, we now require the version to be specified as a command line
argument.
Because we tag the version in the docker image tag as bitcoin-core:XX
but the binary internally is located under /opt/bitcoin-XX.Y/, we
manually set Y to 0.
Because the bitcoind mirror is extremely slow, we spend at least 2 to 3
minutes of each bitcoind related test on just downloading the binary. We
can achieve the same result by just pulling the docker image and
extracting the binary from that.
