fix!: stop implicitly using DOCKER_USERNAME/DOCKER_PASSWORD secrets for image pulls (#1007)

## Background

`DOCKER_USERNAME` and `DOCKER_PASSWORD` are commonly used by workflows as ordinary secrets for logging in to a private registry and pushing images. However, the runner also treated these secret names as implicit Docker pull credentials.

These credentials carry no registry information, but they were attached to every pull unconditionally. As a result, a user who configured `DOCKER_USERNAME` / `DOCKER_PASSWORD` secrets for their private registry (e.g. to push images) would have those same credentials sent to Docker Hub when pulling a public image, causing the pull to fail with authentication failure.

## Changes

- Stop using `DOCKER_USERNAME` and `DOCKER_PASSWORD` as implicit pull credentials for job containers.
- Stop injecting `DOCKER_USERNAME` and `DOCKER_PASSWORD` as pull credentials for step containers.

## ⚠️ BREAKING ⚠️

This is a breaking change.

Workflows or runner setups that previously relied on `DOCKER_USERNAME` and `DOCKER_PASSWORD` being implicitly used for Docker image pulls must migrate to an explicit authentication mechanism.

Migration options:

- For private job container images, use `container.credentials`:

```yaml
  jobs:
    build:
      container:
        image: registry.example.com/image:tag
        credentials:
          username: ${{ secrets.REGISTRY_USERNAME }}
          password: ${{ secrets.REGISTRY_PASSWORD }}
```

- For private service container images, use service `credentials`.

- For private `uses: docker://...` or private Docker actions, configure Docker authentication in the runner environment before the job starts. For example, run `docker login` on the runner host.

`DOCKER_USERNAME` and `DOCKER_PASSWORD` can still be used as ordinary workflow secrets, for example with `docker/login-action` before pushing images.

---

Related:

- Fixes #386

---------

Co-authored-by: Nicolas <bircni@icloud.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1007
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Zettat123 <39446+zettat123@noreply.gitea.com>
Co-committed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

act/runner/run_context_test.go

@@ -170,6 +170,38 @@ func TestRunContext_EvalBool(t *testing.T) {
 	}
 }
 
+func TestRunContextHandleCredentialsDoesNotUseDockerSecrets(t *testing.T) {
+	workflow, err := model.ReadWorkflow(strings.NewReader(`
+name: test
+on: push
+jobs:
+  job:
+    runs-on: ubuntu-latest
+    steps: []
+`))
+	require.NoError(t, err)
+
+	rc := &RunContext{
+		Config: &Config{
+			Secrets: map[string]string{
+				"DOCKER_USERNAME": "docker-user",
+				"DOCKER_PASSWORD": "docker-password",
+			},
+			Env: map[string]string{},
+		},
+		Run: &model.Run{
+			JobID:    "job",
+			Workflow: workflow,
+		},
+	}
+
+	// DOCKER_USERNAME/DOCKER_PASSWORD secrets should not be used as implicit job container pull credentials.
+	username, password, err := rc.handleCredentials(t.Context())
+	require.NoError(t, err)
+	assert.Empty(t, username)
+	assert.Empty(t, password)
+}
+
 func TestRunContext_GetBindsAndMounts(t *testing.T) {
 	rctemplate := &RunContext{
 		Name: "TestRCName",