Adds GitHub actions badge

[anskarl]

Adds GitHub actions badge to README.md:

• replaces Circle CI badge from README.md with similar one from GitHub actions
• renames CI flow from 'Build project' to 'build', in order to simply show the label 'build' in the GitHub actions the badge
• disables docker push when 'github.event_name' is 'pull_request'

[anskarl]

In PR #84 GitHub actions where introduced. There is a step in the build process, named as 'Push docker images', in which the images are be pushed to the project docker registry. However, in case of a pull_request event the build process will always going to fail, since the owner of the pull request does not have the credentials to push to the docker registry of the project. Therefore in this PR the workflow keeps enabled 'Push docker images' step, only when the variable github.event_name is not equal to pull_request. In practice, this step is going to be enabled only for pull events, e.g., after accepting the PR and merging to master.

Also it looks like that CircleCI is still triggered on PRs, maybe is some project-dependent setting that should be disabled.