Added dependabot, use docker login action

[christian-kreuzberger-dtx]

Signed-off-by: Christian Kreuzberger christian.kreuzberger@dynatrace.com

• Added dependabot config for automated dependency updates
• Updated CI workflow to use docker login action

[github-actions[bot]]

The following Docker Images have been built:

• Pushed keptncontrib/prometheus-sli-service:0.2.4-dev-PR-59.202102040736 and keptncontrib/prometheus-sli-service:0.2.4-dev-PR-59 (Source: ./)