Add more content on how to use Exec function (testcontainers#1970)

* docs(features): add more content on exec function

* test(docker-test): add the reader from a command execution to the docker tests

* docs(feature): add embedded code blocks to the docs

docker_test.go

@@ -1298,21 +1298,34 @@ func TestContainerWithTmpFs(t *testing.T) {
 
 	path := "/testtmpfs/test.file"
 
-	c, _, err := container.Exec(ctx, []string{"ls", path})
+	// exec_reader_example {
+	c, reader, err := container.Exec(ctx, []string{"ls", path})
 	if err != nil {
 		t.Fatal(err)
 	}
 	if c != 1 {
 		t.Fatalf("File %s should not have existed, expected return code 1, got %v", path, c)
 	}
 
+	buf := new(strings.Builder)
+	_, err = io.Copy(buf, reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// See the logs from the command execution.
+	t.Log(buf.String())
+	// }
+
+	// exec_example {
 	c, _, err = container.Exec(ctx, []string{"touch", path})
 	if err != nil {
 		t.Fatal(err)
 	}
 	if c != 0 {
 		t.Fatalf("File %s should have been created successfully, expected return code 0, got %v", path, c)
 	}
+	// }
 
 	c, _, err = container.Exec(ctx, []string{"ls", path})
 	if err != nil {

docs/features/override_container_command.md

@@ -1,7 +1,9 @@
-# Sending a CMD to a Container
+# Executing commands
 
-If you would like to send a CMD (command) to a container, you can pass it in to
-the container request via the `Cmd` field...
+## Container startup command
+
+By default the container will execute whatever command is specified in the image's Dockerfile. If you would like to send a CMD (command) to a container, you can pass it in to
+the container request via the `Cmd` field. For example:
 
 ```go
 req := ContainerRequest{
@@ -13,3 +15,19 @@ req := ContainerRequest{
 }
 ```
 
+## Executing a command
+
+You can execute a command inside a running container, similar to a `docker exec` call:
+
+<!--codeinclude-->
+[Executing a command](../../docker_test.go) inside_block:exec_example
+<!--/codeinclude-->
+
+This can be useful for software that has a command line administration tool. You can also get the logs of the command execution (from an object that implements the [io.Reader](https://pkg.go.dev/io#Reader) interface). For example:
+
+
+<!--codeinclude-->
+[Command logs](../../docker_test.go) inside_block:exec_reader_example
+<!--/codeinclude-->
+
+This is done this way, because it brings more flexibility to the user, rather than returning a string.