Docker is a tool for containerizing your projects. When we set up the environment using conda on our machine, we often custimize the package combinations to suit our needs, but when we try to run our code on other machines we may not have the same environment configuration, so our program may not run. It is not scalable, and sometimes not possible, to set up environment on other machines.
Docker solves this problem really well. The official site of Docker provides an excellent guide, check that out.