During the build, Docker reads HTTP_PROXY and HTTPS_PROXY from the build arguments. Once built, the container only uses the address if the variable is in its environment, passed at run.
The classic mistake
Configuring it at build time and assuming it holds at run time. The finished image does not keep it. If the program inside the container leaves through the real IP, the variable is missing from the run.
For SOCKS5 on port 20481, the standard variable does not help: the program inside has to know how to speak SOCKS.