Make alsa dummy snd module work in Github CI

[pestophagous]

Refer to commit b99677c

b99677c#r38308927

[pestophagous]

• https://help.github.com/en/actions/creating-actions/dockerfile-support-for-github-actions
• https://help.github.com/en/actions/creating-actions/creating-a-docker-container-action

unclear whether using our own Dockerfile would help, since docker machines use the same kernel as the host, and the github azure kernel might be the issue.

some output from the CI job that makes it seem like kernel "5.0.0-1035-azure" might be implicated:

 + sudo modprobe snd-dummy
 modprobe: FATAL: Module snd-dummy not found in directory /lib/modules/5.0.0-1035-azure
 + true
 + sudo modprobe snd-seq
 modprobe: ERROR: ../libkmod/libkmod-module.c:832 kmod_module_insert_module() could not find module by name='snd_seq'
 modprobe: ERROR: could not insert 'snd_seq': Unknown symbol in module, or unknown parameter (see dmesg)
 modprobe: ERROR: ../libkmod/libkmod-module.c:979 command_do() Error running install command for snd_seq
 modprobe: ERROR: could not insert 'snd_seq': Operation not permitted