Add Dockerfile; add GitHub Actions build

[k7hpn]

1. Add a Dockerfile that builds sealtool and is able to run tests as part of the build
2. Add a GitHub Action to build the project in the Docker container and push an image up to the GitHub Container Registry.

Note: if you want merging this PR to push the first image up to the container registry, please review the instructions in CI.md to set up the necessary Actions variables and a PAT secret so the container image can be pushed up to the container registry (if you are an owner in the hackerfactor/SEAL-C repository then skip the first fork step). The DOCKER.md file that is included in this PR is assuming that the steps in CI.md are taken before this PR is merged.

Missing from this PR

1. The remote test "passes" immediately and the manual test is failing so testing is not working properly. Running the Docker build locally with SIGNMYDATA_APIKEY and SIGNMYDATA_ID environment variables set succeeds (with docker build --secret id=apikey,env=SIGNMYDATA_APIKEY --secret id=signid,env=SIGNMYDATA_ID --no-cache -t sealtool:latest .).
2. Instructions in DOCKER.md on how to follow the same steps in Windows. Doing directory mapping between Windows and Docker Containers is fidgety, I need to do more testing before I'm ready to document that.

[evelynmitchell]

Why are you using the Sourceforge mirror of exiftool, doing a source build and not the Debian package?
libimage-exiftool-perl Trixie version 13.25+dfsg-1

[k7hpn]

I had originally set this up with the Debian package but since I've been consistently running into difficulties with the test suite I decided to follow the directions in BUILD.md and install the latest version of ExifTool following their instructions just to rule out an older version of ExifTool as a possible culprit. Once it passes all the tests reliably it is my intent to attempt to transition to the Debian packaged ExifTool.

[evelynmitchell]

Why do you need a docker login to use debian stable? I believe you can pull the public image without a login.

[k7hpn]

The Docker login step is to authenticate with the GitHub Container Registry so once the next step generates a Docker image it can be pushed up to the registry.