I looks like building the app UI for OS X has been broken since
https://github.com/sshuttle/sshuttle/commit/9eced8d0494e8d44b28db21f5643d824710c07a1
due to the sources.list.do file still referencing the old .png images.

Without this fix the build will stop at:

    do            chicken-tiny.png
    do: Users/elasticdog/sshuttle/src/ui-macos/chicken-tiny.png: no .do file
    do:         Sshuttle VPN.app: got exit code 1
    do:       Sshuttle VPN.app.zip: got exit code 1
    do:     dist: got exit code 1
    do:   ui-macos/all: got exit code 1
    do: all: got exit code 1