small improvements for development guide

[balopat]

As I went through the build the first time (both cloud and local) I ran into two issues:

• my user did not have docker group setup - and that is assumed by the build script, so I mentioned that assumption explicitly in the guide
• for the "local emulation" of the cloud build, a python script is pulled in from the python runtime - which is not an ideal solution necessarily - especially that the old version (the one I switched back to) worked, the new version doesn't, as it got modularized in this commit - I'll put in an issue to have a better fix than just depending on an older version

[aslo]

👍 Thanks for the improvements to the build process!

I'm also not crazy about the way we currently do local emulation of cloud builds. We can tackle this in a separate issue, but I might even prefer to remove it altogether, since maven is the recommended way to build locally. We've already done that in the Jetty repo. @meltsufin what do you think?

[meltsufin]

@balopat Thanks for the improvement and opening that issue to track local build story. This looks good.