Automated Multi Platform Build

[antoniotorres]

I would like to propose the following PR to enable a better way to build and publish the docker images to Docker Hub. This PR also adds other steps that are necessary to make this image multi-platform and others are to permit user to contribute the project easily.

What has changed:

• Add Github Action Workflow to build on multiple platforms
• Add simple README
• Add dependabot updater for github actions dependencies
• Add Trivy step to look for vulnerabilities of the docker image
• Change GOSU to use documentation example
• Change adoptopenjdk to eclipse-temurin

What reviewers should know:

Please note that to be able to publish automatically to dockerhub, you must add the following secrets to the Github repo:

DOCKERHUB_USERNAME
DOCKERHUB_TOKEN

To build locally install and use buildx to be able to build for ARM or other platforms.

[gunterze]

Providing multi-platform images on each release seems for me sufficient. s. dcm4che/dcm4chee-arc-light#3487
(What's the intention for building ffmpeg yourself and not just install the pre-build binary package for the particular platform?)