feat: add context info to docker build errors (#3920)

its too hard to debug docker build issues... sometimes is just a typo in
the binary name, and you might end debugging it for way too long...

this prints the full path to the build context (so, locally at least,
you can cd into it) and also all the files available there when the
error seems to be one of the "file not found" kind.

Hopefully this helps fixing things easier :)

closes #3912

Signed-off-by: Carlos A Becker <caarlos0@users.noreply.github.com>

internal/pipe/docker/docker.go

@@ -2,6 +2,7 @@ package docker
 
 import (
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"sort"
@@ -198,6 +199,26 @@ func process(ctx *context.Context, docker config.Docker, artifacts []*artifact.A
 
 	log.Info("building docker image")
 	if err := imagers[docker.Use].Build(ctx, tmp, images, buildFlags); err != nil {
+		if strings.Contains(err.Error(), "file not found") || strings.Contains(err.Error(), "not found: not found") {
+			var files []string
+			_ = filepath.Walk(tmp, func(path string, info fs.FileInfo, err error) error {
+				if info.IsDir() {
+					return nil
+				}
+				files = append(files, info.Name())
+				return nil
+			})
+			return fmt.Errorf(`seems like you tried to copy a file that is not available in the build context.
+
+Here's more information about the build context:
+
+dir: %q
+files in that dir:
+ %s
+
+Previous error:
+%w`, tmp, strings.Join(files, "\n "), err)
+		}
 		return err
 	}
 

internal/pipe/docker/docker_test.go

@@ -436,6 +436,20 @@ func TestRunPipe(t *testing.T) {
 			pubAssertError:      shouldNotErr,
 			manifestAssertError: shouldNotErr,
 		},
+		"wrong binary name": {
+			dockers: []config.Docker{
+				{
+					ImageTemplates: []string{
+						registry + "goreleaser/wrong_bin_name:v1",
+					},
+					Goos:       "linux",
+					Goarch:     "amd64",
+					Dockerfile: "testdata/Dockerfile.wrongbin",
+				},
+			},
+			assertError:       shouldErr("seems like you tried to copy a file that is not available in the build context"),
+			assertImageLabels: noLabels,
+		},
 		"templated-dockerfile-invalid": {
 			dockers: []config.Docker{
 				{

internal/pipe/docker/testdata/Dockerfile.wrongbin

@@ -0,0 +1,3 @@
+FROM alpine
+# a typo:
+ADD mybyn /