Introduce Dependabot for Docker and GitHub Actions

[programmer04]

As mentioned in #111 (comment) introduce automated updates for base images in Dockerfile and GitHub Actions used in CI. It will check for updates every day and create PRs automatically for us - keeping them up to date should allow avoiding most security vulnerabilities

[Dentrax]

I think setting interval to weekly would much efficient since we get less spammed by dependabot notifications.

[programmer04]

My reasoning was that in Dockerfile only FROM alpine:3.16.0 will be tracked and Alpine is not releasing so often

GitHub Actions in files test-pr-buildx.yml and release-buildx.yml are pinned only by major versions so they shouldn't change so often too.

Hence by checking it every day, mostly it won't update anything. But when new versions come up, PRs will be created almost immediately (max one day delay).