README.md: Consolidate Docker image build instructions
README.md: Consolidate Docker image build instructions

Consolidate the Docker image build instructions into the "Building Scylla"
section of the README instead of having it in a separate section in a different
place of the file.

Message-Id: <dbda478b22b9caf6bab93b473529313df0fbd06c@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla