Ubuntu 20.04 as a base image for CVAT Dockerfile

[azhavoro]

Resolve #1930 resolve #1817 resolve #1457

Updated Ubuntu 20.04 as base image for CVAT container

Motivation and context

How has this been tested?

Manually tested + CI tests

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@azhavoro , need to resolve the conflict.

[coveralls]

Pull Request Test Coverage Report for Build 7230

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 10 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.7%) to 70.399%

---

Files with Coverage Reduction	New Missed Lines	%
datumaro/datumaro/cli/main.py	1	0%
datumaro/datumaro/plugins/accuracy_checker_plugin/details/ac.py	1	4.17%
datumaro/datumaro/plugins/accuracy_checker_plugin/launcher.py	1	25.0%
datumaro/datumaro/util/init.py	1	78.49%
datumaro/datumaro/components/project.py	2	80.37%
datumaro/datumaro/plugins/openvino_launcher.py	4	3.95%

Totals
Change from base Build 7219:	0.7%
Covered Lines:	12040
Relevant Lines:	16700

---

💛 - Coveralls

[nmanovic]

Do we need to update contribution guide?

[nmanovic]

Please add a line into CHANGELOG