Add a GitHub Action for a container image build

[git-developer]

This PR adds a GitHub Action for a container image build.

Currently, the container image is provided in a separate project git-developer/ti-cc-tool. I'd like to contribute it to this project.

The building blocks are:

1. A Dockerfile that contains the build instructions. By default, sources are taken from the local filesystem and all extra features are included. This can be configured by setting the build-arg REPO_URL (e.g. to an URL).
2. A run shell script that allows firmware files as URL or local file, in native (bin/hex) and zipped format. It is used as entrypoint for the container image.
3. A GitHub Action workflow .github/workflows/image-build.yml that starts the image build. It is currently configured to trigger a build on creation of a release (via the GitHub Web-UI) and additionally on pushes to master. The image is created for several platforms in parallel (e.g. amd64and arm/v7) and uploaded to the GitHub package registry of this project.

I created a release called demo-1 in my fork for you to get an impression what happens and how it looks like:

• Actions
• Releases
• Packages

This PR probably needs some discussions and finetuning. Please add your comments so that we can elaborate on how it can be adapted to your requirements.

User documentation is not part of this PR yet. I suggest to merge the README of git-developer/ti-cc-tool with this project.