Change DockerFile to ubuntu 20.04; correct OS check issue; run build automatically

[xdddduo]

I fixed two issues I met when installing avr using docker:

1. Changed environment to be ubuntu 20.04 (otherwise would have problem finding distutils, which is already deprecated in Python 3.12, the version Ubuntu 24.04 is using)
2. Added a line to modify the OS checking in Dockerfile before building build.sh (otherwise would recheck OS though it is already using ubuntu. And since docker shares the kernal with the host, there would be no 'ubuntu' when doing uname -a), leading to excuting yum in original version)

[xdddduo]

I also revised DockerFile to resolve the prompt of time zone. So the latest version does need to run build.sh by hand.

[aman-goel]

@xdddduo Thanks for these changes!