feat: add support for building docker actions with private registries (#1557)

This commit adds a new `LoadDockerAuthConfigs` function, which loads all
registry auths that are configured on the host and sends them with the build
command to the docker daemon.

This is needed in case act builds a docker action and the images referenced in
that docker action are located on private registries or otherwise require
authentication (e.g. to get a higher rate limit).

The code is adapted from how the docker cli works:
https://github.com/docker/cli/blob/257ff41304bf121bdf1acdf00a1c7a896ed038d1/cli/command/image/build.go#L323-L332

Co-authored-by: Markus Wolf <mail@markus-wolf.de>

Co-authored-by: Markus Wolf <mail@markus-wolf.de>

pkg/container/docker_build.go

@@ -41,9 +41,10 @@ func NewDockerBuildExecutor(input NewDockerBuildExecutorInput) common.Executor {
 
 		tags := []string{input.ImageTag}
 		options := types.ImageBuildOptions{
-			Tags:     tags,
-			Remove:   true,
-			Platform: input.Platform,
+			Tags:        tags,
+			Remove:      true,
+			Platform:    input.Platform,
+			AuthConfigs: LoadDockerAuthConfigs(ctx),
 		}
 		var buildContext io.ReadCloser
 		if input.Container != nil {