fix: allow relocating the tool cache and mounting over runner paths (#1122)

1. Docker rejects two mounts on one target, so a `container.volumes:` or `--volume` aimed at `/opt/hostedtoolcache` failed the job with `Duplicate mount point`. Job and service volumes now displace the mount on the same path, and `name:/target:ro` no longer mounts read-write at the literal path `/target:ro`.
1. Setting `RUNNER_TOOL_CACHE` only changed what the variable said, the cache stayed where it was, so tools writing to it landed outside the mount and `${{ runner.tool_cache }}` disagreed with the variable. It now relocates the cache. Leaving it unset behaves as before.
1. Unknown `config.yaml` keys now warn instead of being dropped without a trace.

Fixes https://gitea.com/gitea/runner/issues/813

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1122
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

internal/pkg/config/config.example.yaml

@@ -167,7 +167,11 @@ container:
     enable_ipv6: false # Omit to use Docker's default (IPv6 disabled). Enabling it requires dockerd started with --ipv6.
   # Whether to use privileged mode or not when launching task containers (privileged mode is required for Docker-in-Docker).
   privileged: false
-  # Any other options to be used when the container is started (e.g., --add-host=my.gitea.url:host-gateway).
+  # Any other options to be used when the container is started, for example:
+  #   options: --add-host=my.gitea.url:host-gateway
+  # A volume declared here replaces the one the runner mounts on the same container path, so the
+  # tool cache can be kept on the host. Its source must also be allowed by valid_volumes below:
+  #   options: --volume /host/toolcache:/opt/hostedtoolcache
   options:
   # The parent directory of a job's working directory.
   # NOTE: There is no need to add the first '/' of the path as runner will add it automatically.

internal/pkg/config/config.go

@@ -4,6 +4,7 @@
 package config
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"maps"
@@ -143,6 +144,7 @@ func LoadDefault(file string) (*Config, error) {
 		if err := yaml.Unmarshal(content, cfg); err != nil {
 			return nil, fmt.Errorf("parse config file %q: %w", file, err)
 		}
+		warnUnknownKeys(file, content)
 		definedRunnerKeys, err = definedRunnerConfigKeys(content)
 		if err != nil {
 			return nil, fmt.Errorf("parse config file %q for defaults metadata: %w", file, err)
@@ -288,6 +290,21 @@ func LoadDefault(file string) (*Config, error) {
 	return cfg, nil
 }
 
+// warnUnknownKeys reports keys the config does not define, which are otherwise ignored
+// without a trace. It only warns, so a config carrying keys from another runner version
+// still loads.
+func warnUnknownKeys(file string, content []byte) {
+	decoder := yaml.NewDecoder(bytes.NewReader(content))
+	decoder.KnownFields(true)
+
+	var typeErr *yaml.TypeError
+	if err := decoder.Decode(&Config{}); errors.As(err, &typeErr) {
+		for _, message := range typeErr.Errors {
+			log.Warnf("config file %q: %s, it will be ignored", file, message)
+		}
+	}
+}
+
 func definedRunnerConfigKeys(content []byte) (map[string]bool, error) {
 	var root yaml.Node
 	if err := yaml.Unmarshal(content, &root); err != nil {

internal/pkg/config/config_test.go

@@ -9,6 +9,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/sirupsen/logrus/hooks/test"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -197,6 +198,21 @@ func TestLoadDefault_MalformedYAMLReturnsParseError(t *testing.T) {
 	assert.NotContains(t, err.Error(), "defaults metadata")
 }
 
+func TestLoadDefault_WarnsOnUnknownKeysButStillLoads(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	require.NoError(t, os.WriteFile(path, []byte("container:\n  volumes:\n    - /host:/ctr\n  privileged: true\n"), 0o600))
+
+	hook := test.NewGlobal()
+	defer hook.Reset()
+
+	cfg, err := LoadDefault(path)
+	require.NoError(t, err)
+	assert.True(t, cfg.Container.Privileged)
+	require.Len(t, hook.Entries, 1)
+	assert.Contains(t, hook.LastEntry().Message, "field volumes not found")
+}
+
 func TestContainerNetworkCreateOptions(t *testing.T) {
 	// Verify that the enable_ipv4/enable_ipv6 YAML keys unmarshal into the *bool fields,
 	// distinguishing an explicit true/false from an omitted key (nil). A nil here is