Update docker images to Ubuntu 20.04

[sjoelund]

No description provided.

[AnHeuermann]

@sjoelund I added some dev containers. If you continue to work on this please also update the devcontainer.json files, e.g. https://github.com/OpenModelica/OpenModelica/blob/master/.devcontainer/build-deps-v1.16.4/devcontainer.json#L7.

  // Build and tag base Docker image from .CI/cache/Dockerfile
-  "initializeCommand": "docker build --tag build-deps-v1.16.4-base ${localWorkspaceFolder}/.CI/cache/",
+  "initializeCommand": "docker build --tag build-deps-v1.21.1-base ${localWorkspaceFolder}/.CI/cache/",

  // Or use a Dockerfile or Docker Compose file. More info: https://containers.dev/guide/dockerfile
  "build": {
    "dockerfile": "Dockerfile",
    "args": {
-        "BASE_IMAGE": "build-deps-v1.16.4-base",
+        "BASE_IMAGE": "build-deps-v1.21.1-base",
        // On Windows USERNAME is set, on Linux USER is set
        // We hope only one is set
        "USERNAME": "${localEnv:USER}${localEnv:USERNAME}"
    }
  },

[CLAassistant]

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