Misc improvements

• fix --build-dirs-path option
• document --copy option
• allow using custom container engine
• support running from inside distrobox container
• ignore notification failures

p.s.: Currently I am using a similar approach (https://github.com/cgzones/container-deb-builder), with the main difference of build dependencies being installed into the container instead of the image, which makes the image re-usable for different source packages but grants the build process network access.