Allow configuration of default platform if no runs-on labels match

internal/pkg/config/config.example.yaml

@@ -41,6 +41,8 @@ runner:
     - "ubuntu-latest:docker://gitea/runner-images:ubuntu-latest"
     - "ubuntu-22.04:docker://gitea/runner-images:ubuntu-22.04"
     - "ubuntu-20.04:docker://gitea/runner-images:ubuntu-20.04"
+  # If no labels match the job's runs-on, this will be used
+  default_platform: "gitea/runner-images:ubuntu-latest"
 
 cache:
   # Enable cache server to use actions/cache.
@@ -70,7 +72,7 @@ container:
   # And other options to be used when the container is started (eg, --add-host=my.gitea.url:host-gateway).
   options:
   # The parent directory of a job's working directory.
-  # NOTE: There is no need to add the first '/' of the path as act_runner will add it automatically. 
+  # NOTE: There is no need to add the first '/' of the path as act_runner will add it automatically.
   # If the path starts with '/', the '/' will be trimmed.
   # For example, if the parent directory is /path/to/my/dir, workdir_parent should be path/to/my/dir
   # If it's empty, /workspace will be used.

internal/pkg/config/config.go

@@ -31,6 +31,7 @@ type Runner struct {
 	FetchTimeout    time.Duration     `yaml:"fetch_timeout"`    // FetchTimeout specifies the timeout duration for fetching resources.
 	FetchInterval   time.Duration     `yaml:"fetch_interval"`   // FetchInterval specifies the interval duration for fetching resources.
 	Labels          []string          `yaml:"labels"`           // Labels specify the labels of the runner. Labels are declared on each startup
+	DefaultPlatform string            `yaml:"default_platform"` // DefaultPlatform specify the image to be used if no runs-on matches our labels
 }
 
 // Cache represents the configuration for caching.