FIX: docker build error for ludwig-gpu

[jimthompson5802]

Update base image to Ubuntu 18.04 to 20.04 to resolve this error in the build for for docker image ludwig-gpu.

#12 7.755 Unpacking cuda-keyring (1.0-1) ...
#12 7.779 Setting up cuda-keyring (1.0-1) ...
#12 7.861 Warning: apt-key output should not be parsed (stdout is not a terminal)
#12 7.881 Executing: /tmp/apt-key-gpghome.JznHdaISEj/gpg.1.sh --keyserver keyserver.ubuntu.com --recv-keys A4B469963BF863CC F60F4B3D7FA2AF80
#12 7.943 gpg: keyserver receive failed: Cannot assign requested address

With EOSS for 18.04, it appears some of of the Ubuntu resources are being decommissioned: https://askubuntu.com/questions/1481617/randomly-get-gpg-keyserver-receive-failed-cannot-assign-requested-address

[github-actions]

Unit Test Results

  6 files  +  1    6 suites  +1   37m 45s ⏱️ + 8m 45s
31 tests ±  0  26 ✔️ ±  0    5 💤 ±0  0 ❌ ±0 
82 runs  +19  66 ✔️ +17  16 💤 +2  0 ❌ ±0 

Results for commit 374cc96. ± Comparison against base commit ef2c14a.