Automate docker image builds

[goatgoose]

Problem:

Currently the docker images used in CI are re-built manually whenever we add something to them. Manually building the docker images and uploading them to ECR is an involved process, so productivity would increase if this process was automated.

Additionally, all of the tooling used in CI is fixed until the docker image is rebuilt, even if the install scripts download the latest version.

Solution:

Automate the process of building the docker images used in CI with Github actions.

Requirements / Acceptance Criteria:

• The CI docker images are regularly rebuilt without developer involvement.
• New versions of all software used in the images are automatically installed when rebuilding, avoiding the need to track this manually (Track pinned versions of CI tooling #4591).
• The docker images are moved to Github so they can be easily modified: Move CI docker images to the s2n-tls repository #4479