Docker needs gcc-9

[slspencer]

Hi @mhitza !!!

Can you install gcc-9 in Docker to correct this problem? Thanks!!!!

Background:
Both Ubuntu 18.04 and the default Docker build use gcc-8, and Ubuntu 20.04 (Ubuntu-latest) uses gcc-9.
Our weekly release is built on Ubuntu-latest (gcc-9) then your linuxdeployqt pulls it into Docker (gcc-8). The build fails with an error message about not finding gcc-9. see message below.
Our other workflows allow us to build with Ubuntu 18.04 so there isn't a problem.

Can you install gcc-9 in Docker to correct this problem? Then we can upgrade all of our workflows to Ubuntu-latest, and resume our weekly builds.

Status: Downloaded newer image for mhitza/linuxdeployqt:5.13.2
docker: Error response from daemon: OCI runtime create failed: container_linux.go:367: starting container process caused: exec: "gcc:9.3": executable file not found in $PATH: unknown.
time="2021-03-22T00:58:53Z" level=error msg="error waiting for container: context canceled"
Error: Process completed with exit code 127.

link to this build message: https://github.com/FashionFreedom/Seamly2D/runs/2162115087?check_suite_focus=true