Docker build cannot download from libwebsockets.org

The libwebsockets.org website is occasionally down, and the project has stated that they are having trouble keeping the site going, recommending instead that people download from GitHub:

https://github.com/warmcat/libwebsockets/issues/2841#issuecomment-1497017643

The site is now either down or responding in a way that prevents the guacd Docker image from building libwebsockets:

...
#8 278.0 Cloning into 'libwebsockets'...
#8 280.7 error: RPC failed; curl 18 HTTP/2 stream 3 was reset
#8 280.7 fetch-pack: unexpected disconnect while reading sideband packet
#8 280.7 fatal: early EOF
#8 280.7 fatal: fetch-pack: invalid index-pack output

We should point the image build at GitHub instead, as advised.