Docker auth (#891)

* feat: read docker credentials from local docker config

* fix: url.Parse requires protocol

Co-authored-by: Björn Brauer <[email protected]>

* fix: docker decides by the existence of . or : if...

... the image is in a custom registry or not.

Co-authored-by: Björn Brauer <[email protected]>

* fix: make docker hostname detection more robust

* test: mock docker config for getImagePullOptions test

By default github actions have a docker config set with a token to pull
images from docker hub.

Co-authored-by: Markus Wolf <[email protected]>

Co-authored-by: Markus Wolf <[email protected]>

pkg/container/testdata/docker-pull-options/config.json

@@ -0,0 +1,7 @@
+{
+  "auths": {
+          "https://index.docker.io/v1/": {
+                  "auth": "dXNlcm5hbWU6cGFzc3dvcmQK"
+          }
+  }
+}