Bump ubuntu from 18.04 to 20.04 (#113)

Bumps ubuntu from 18.04 to 20.04.

---
updated-dependencies:
- dependency-name: ubuntu
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Dockerfile.cuda

@@ -4,7 +4,7 @@ ARG FFMPEG_VERSION=5.0
 
 ##############################
 # Build the NGINX-build image.
-FROM ubuntu:18.04 as build-nginx
+FROM ubuntu:20.04 as build-nginx
 ARG NGINX_VERSION
 ARG NGINX_RTMP_VERSION
 ARG MAKEFLAGS="-j4"