Since the Python HTTP server can only serve files from the current directory that it runs in, without explicitly configuring this correctly our container would show files out of the / directory. To work around this, we include WORKDIR $SRV_ROOT into the Dockerfile which changes our working directory to the one that will contain the files we want to serve up. A thing to note about this command is that you can reuse it as many times as you want and it applies to any subsequent commands in the Dockerfile (such as RUN or CMD).