test: fix e2e test for privileged builds (docker#10873)

We cannot guarantee the exact value of `CapEff` across
environments, and this test has started failing some places,
e.g. Docker Desktop, and now GitHub Actions (likely due to
a kernel upgrade on the runners or similar).

By setting `privileged: true` on the build, we're asking for
the `security.insecure` entitlement on the build. A safe
assumption is that will include `CAP_SYS_ADMIN`, which won't
be present otherwise, so mask the `CapEff` value and check
for that.

It's worth noting that realistically, the build won't even
be able to complete without the correct entitlement, since the
`Dockerfile` uses `RUN --security=insecure`, so this is really
an additional sanity check.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>

pkg/e2e/build_test.go

@@ -19,7 +19,9 @@ package e2e
 import (
 	"fmt"
 	"net/http"
+	"regexp"
 	"runtime"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -366,10 +368,21 @@ func TestBuildPrivileged(t *testing.T) {
 	})
 
 	t.Run("use build privileged mode to run insecure build command", func(t *testing.T) {
-		res := c.RunDockerComposeCmdNoCheck(t, "--project-directory", "fixtures/build-test/privileged", "build")
-		assert.NilError(t, res.Error, res.Stderr())
-		res.Assert(t, icmd.Expected{Out: "CapEff:\t0000003fffffffff"})
-
+		res := c.RunDockerComposeCmd(t, "--project-directory", "fixtures/build-test/privileged", "build")
+		capEffRe := regexp.MustCompile("CapEff:\t([0-9a-f]+)")
+		matches := capEffRe.FindStringSubmatch(res.Stdout())
+		assert.Equal(t, 2, len(matches), "Did not match CapEff in output, matches: %v", matches)
+
+		capEff, err := strconv.ParseUint(matches[1], 16, 64)
+		assert.NilError(t, err, "Parsing CapEff: %s", matches[1])
+
+		// NOTE: can't use constant from x/sys/unix or tests won't compile on macOS/Windows
+		// #define CAP_SYS_ADMIN        21
+		// https://github.com/torvalds/linux/blob/v6.1/include/uapi/linux/capability.h#L278
+		const capSysAdmin = 0x15
+		if capEff&capSysAdmin != capSysAdmin {
+			t.Fatalf("CapEff %s is missing CAP_SYS_ADMIN", matches[1])
+		}
 	})
 }