docker: report unhealthy in unsupported Windows (#5356)

On Windows, Nomad only supports Windows containers, so report as
unhealthy otherwise.

drivers/docker/fingerprint.go

@@ -2,6 +2,7 @@ package docker
 
 import (
 	"context"
+	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -158,6 +159,19 @@ func (d *Driver) buildFingerprint() *drivers.Fingerprint {
 
 		fp.Attributes["driver.docker.runtimes"] = pstructs.NewStringAttribute(
 			strings.Join(runtimeNames, ","))
+		fp.Attributes["driver.docker.os_type"] = pstructs.NewStringAttribute(dockerInfo.OSType)
+
+		if runtime.GOOS == "windows" && dockerInfo.OSType == "linux" {
+			if d.fingerprintSuccessful() {
+				d.logger.Warn("detected Linux docker containers on Windows; only Windows containers are supported")
+			}
+
+			d.setFingerprintFailure()
+			return &drivers.Fingerprint{
+				Health:            drivers.HealthStateUnhealthy,
+				HealthDescription: "Docker is configured with Linux containers; only Windows containers are supported",
+			}
+		}
 	}
 
 	d.setFingerprintSuccess()

drivers/docker/fingerprint_test.go

@@ -0,0 +1,27 @@
+package docker
+
+import (
+	"testing"
+
+	"github.com/hashicorp/nomad/client/testutil"
+	"github.com/hashicorp/nomad/helper/testlog"
+	"github.com/hashicorp/nomad/plugins/drivers"
+	tu "github.com/hashicorp/nomad/testutil"
+	"github.com/stretchr/testify/require"
+)
+
+// TestDockerDriver_FingerprintHealth asserts that docker reports healthy
+// whenever Docker is supported.
+//
+// In Linux CI and AppVeyor Windows environment, it should be enabled.
+func TestDockerDriver_FingerprintHealth(t *testing.T) {
+	if !tu.IsCI() {
+		t.Parallel()
+	}
+	testutil.DockerCompatible(t)
+
+	d := NewDockerDriver(testlog.HCLogger(t)).(*Driver)
+
+	fp := d.buildFingerprint()
+	require.Equal(t, drivers.HealthStateHealthy, fp.Health)
+}