Ubuntu base used (18.04) need to be replaced with newer Ubuntu 20.04 LTS

[bsriramprasad]

Describe the feature

Dockerfile used to build the AWS Gstreamer KVSSINK element need to use newer ubuntu base image say 20.04
Current base image 18.04 will reach end of standard support by April 2023.

Refer : https://github.com/awslabs/amazon-kinesis-video-streams-producer-sdk-cpp/blob/master/.github/workflows/ci.yml

• Use base ubuntu (instead of architecture specific ubuntu)
• Install appropriate cross compile tool chain
• Build the Gstreamer element
• Copy only the relevant binary into a scratch ubuntu image to reduce size to bare minimum.

This process would ensure we are in sync with the process AWS is employing in their SDK repo.

Added value

This would ensure that the application image we build does not carry any known vulnerabilities identified between 18.04 and 20.04 (Ref: https://wiki.ubuntu.com/Releases)