fix: handle dockerignore exclusions properly (testcontainers#2476)

* chore: only include the dockerignore if it contains ignore files

* fix: the inclusions must be relative to the context

* docs: document the dockerignore feature

* chore: only include the dockerignore file if it exists

Author: mdelapenya

container.go

@@ -222,13 +222,17 @@ func (c *ContainerRequest) GetContext() (io.Reader, error) {
 	}
 	c.Context = abs
 
-	excluded, err := parseDockerIgnore(abs)
+	dockerIgnoreExists, excluded, err := parseDockerIgnore(abs)
 	if err != nil {
 		return nil, err
 	}
 
-	dockerIgnoreLocation := filepath.Join(abs, ".dockerignore")
-	includes = append(includes, dockerIgnoreLocation, c.GetDockerfile())
+	if dockerIgnoreExists {
+		// only add .dockerignore if it exists
+		includes = append(includes, filepath.Join(".dockerignore"))
+	}
+
+	includes = append(includes, c.GetDockerfile())
 
 	buildContext, err := archive.TarWithOptions(
 		c.Context,
@@ -241,18 +245,24 @@ func (c *ContainerRequest) GetContext() (io.Reader, error) {
 	return buildContext, nil
 }
 
-func parseDockerIgnore(targetDir string) ([]string, error) {
+// parseDockerIgnore returns if the file exists, the excluded files and an error if any
+func parseDockerIgnore(targetDir string) (bool, []string, error) {
 	// based on https://github.com/docker/cli/blob/master/cli/command/image/build/dockerignore.go#L14
 	fileLocation := filepath.Join(targetDir, ".dockerignore")
 	var excluded []string
+	exists := false
 	if f, openErr := os.Open(fileLocation); openErr == nil {
+		defer f.Close()
+
+		exists = true
+
 		var err error
 		excluded, err = ignorefile.ReadAll(f)
 		if err != nil {
-			return excluded, fmt.Errorf("error reading .dockerignore: %w", err)
+			return true, excluded, fmt.Errorf("error reading .dockerignore: %w", err)
 		}
 	}
-	return excluded, nil
+	return exists, excluded, nil
 }
 
 // GetBuildArgs returns the env args to be used when creating from Dockerfile

container_ignore_test.go

@@ -11,23 +11,32 @@ import (
 func TestParseDockerIgnore(t *testing.T) {
 	testCases := []struct {
 		filePath         string
+		exists           bool
 		expectedErr      error
 		expectedExcluded []string
 	}{
 		{
 			filePath:         "./testdata/dockerignore",
 			expectedErr:      nil,
+			exists:           true,
 			expectedExcluded: []string{"vendor", "foo", "bar"},
 		},
 		{
 			filePath:         "./testdata",
 			expectedErr:      nil,
+			exists:           true,
 			expectedExcluded: []string{"Dockerfile", "echo.Dockerfile"},
 		},
+		{
+			filePath:         "./testdata/data",
+			expectedErr:      nil,
+			expectedExcluded: nil, // it's nil because the parseDockerIgnore function uses the zero value of a slice
+		},
 	}
 
 	for _, testCase := range testCases {
-		excluded, err := parseDockerIgnore(testCase.filePath)
+		exists, excluded, err := parseDockerIgnore(testCase.filePath)
+		assert.Equal(t, testCase.exists, exists)
 		assert.Equal(t, testCase.expectedErr, err)
 		assert.Equal(t, testCase.expectedExcluded, excluded)
 	}

docs/features/build_from_dockerfile.md

@@ -33,7 +33,6 @@ You can specify them like:
 If you would like to send a build context that you created in code (maybe you have a dynamic Dockerfile), you can
 send the build context as an `io.Reader` since the Docker Daemon accepts it as a tar file, you can use the [tar](https://golang.org/pkg/archive/tar/) package to create your context.
 
-
 To do this you would use the `ContextArchive` attribute in the `FromDockerfile` struct.
 
 ```go
@@ -52,6 +51,15 @@ fromDockerfile := testcontainers.FromDockerfile{
 **Please Note** if you specify a `ContextArchive` this will cause _Testcontainers for Go_ to ignore the path passed
 in to `Context`.
 
+## Ignoring files in the build context
+
+The same as Docker has a `.dockerignore` file to ignore files in the build context, _Testcontainers for Go_ also supports this feature.
+A `.dockerignore` living in the root of the build context will be used to filter out files that should not be sent to the Docker daemon.
+The `.dockerignore` file won't be sent to the Docker daemon either.
+
+!!! note
+    At this moment, _Testcontainers for Go_ does not support Dockerfile-specific `.dockerignore` files.
+
 ## Images requiring auth
 
 If you are building a local Docker image that is fetched from a Docker image in a registry requiring authentication