Add support for multible registrys (#23)

if normal settings `registry`, `username`... is set

that's used as default, else first logins entry is used.

```yml
settings:
  logins:
    - registry: https://codeberg.org
      username: "6543"
      password: geheim
    - registry: https://index.docker.io/v1/
      username: a6543
      password: anders_geheim
```

close #18

Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/23
Reviewed-by: anbraten <anbraten@noreply.codeberg.org>

plugin/docker.go

@@ -10,19 +10,6 @@ import (
 	"github.com/urfave/cli/v2"
 )
 
-// helper function to create the docker login command.
-func commandLogin(login Login) *exec.Cmd {
-	if login.Email != "" {
-		return commandLoginEmail(login)
-	}
-	return exec.Command(
-		dockerExe, "login",
-		"-u", login.Username,
-		"-p", login.Password,
-		login.Registry,
-	)
-}
-
 // helper to check if args match "docker pull <image>"
 func isCommandPull(args []string) bool {
 	return len(args) > 2 && args[1] == "pull"