integration-cli: TestConcurrentPush: refactor to improve failure logs

This tests, when failing, only produced a non-informative "exit status 1",
which limits investigating why it failed.

This patch:

- Rewrites the parallel pushes to use an error-group, and asserts each
  push to get the failure output of the command.
- Simplifies the Dockerfile that's used for building the test-image,
  removing steps that were not needed for the test.
- Adds a cleanup step to make sure the images are deleted after the
  test, or if the test fails (although the test-suite should already
  handle this).

Before this, a failure looked like:

    make BIND_DIR=. TEST_FILTER='TestConcurrentPush' test-integration-cli
    === FAIL: arm64.integration-cli TestDockerRegistrySuite/TestConcurrentPush (5.49s)
        docker_cli_push_test.go:159: assertion failed: error is not nil: exit status 1: concurrent push failed with error: exit status 1
        check_test.go:476: [dfa779e71fdf8] daemon is not started
        --- FAIL: TestDockerRegistrySuite/TestConcurrentPush (5.49s)

With this patch applied:

    make BIND_DIR=. TEST_FILTER='TestConcurrentPush' test-integration-cli
    === FAIL: arm64.integration-cli TestDockerRegistrySuite/TestConcurrentPush (2.47s)
        docker_cli_push_test.go:156: assertion failed:
            Command:  /usr/local/cli-integration/docker push 127.0.0.1:5000/dockercli/busybox:push2nosuch
            ExitCode: 1
            Error:    exit status 1
            Stdout:   The push refers to repository 127.0.0.1:5000/dockercli/busybox

            Stderr:   tag does not exist: 127.0.0.1:5000/dockercli/busybox:push2nosuch

            Failures:
            ExitCode was 1 expected 0
            Expected no error
        docker_cli_push_test.go:160: assertion failed: error is not nil: exit status 1
        check_test.go:476: [db77ef03a8fd8] daemon is not started
        --- FAIL: TestDockerRegistrySuite/TestConcurrentPush (2.47s)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration-cli/docker_cli_push_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/moby/moby/api/types/versions"
 	"github.com/moby/moby/v2/integration-cli/cli"
 	"github.com/moby/moby/v2/integration-cli/cli/build"
+	"golang.org/x/sync/errgroup"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/icmd"
@@ -134,41 +135,38 @@ func (s *DockerRegistrySuite) TestConcurrentPush(c *testing.T) {
 	var repos []string
 	for _, tag := range []string{"push1", "push2", "push3"} {
 		repo := fmt.Sprintf("%v:%v", imgRepo, tag)
-		buildImageSuccessfully(c, repo, build.WithDockerfile(fmt.Sprintf(`
-	FROM busybox
-	ENTRYPOINT ["/bin/echo"]
-	ENV FOO foo
-	ENV BAR bar
-	CMD echo %s
-`, repo)))
+		buildImageSuccessfully(c, repo, build.WithDockerfile(fmt.Sprintf("FROM busybox\nCMD echo hello from %s\n", repo)))
 		repos = append(repos, repo)
 	}
 
+	cleanup := func() {
+		args := append([]string{"rmi", "--force"}, repos...)
+		cli.DockerCmd(c, args...)
+	}
+	defer cleanup()
+
 	// Push tags, in parallel
-	results := make(chan error, len(repos))
-
+	var eg errgroup.Group
 	for _, repo := range repos {
-		go func(repo string) {
+		eg.Go(func() error {
 			result := icmd.RunCommand(dockerBinary, "push", repo)
-			results <- result.Error
-		}(repo)
-	}
-
-	for range repos {
-		err := <-results
-		assert.NilError(c, err, "concurrent push failed with error: %v", err)
+			// check the result, but don't fail immediately (result.Assert)
+			// to prevent the errgroup from not completing.
+			assert.Check(c, result.Equal(icmd.Success))
+			return result.Error
+		})
 	}
+	assert.NilError(c, eg.Wait())
 
 	// Clear local images store.
-	args := append([]string{"rmi"}, repos...)
-	cli.DockerCmd(c, args...)
+	cleanup()
 
 	// Re-pull and run individual tags, to make sure pushes succeeded
 	for _, repo := range repos {
 		cli.DockerCmd(c, "pull", repo)
 		cli.DockerCmd(c, "inspect", repo)
 		out := cli.DockerCmd(c, "run", "--rm", repo).Combined()
-		assert.Equal(c, strings.TrimSpace(out), "/bin/sh -c echo "+repo)
+		assert.Equal(c, strings.TrimSpace(out), "hello from "+repo)
 	}
 }