docs: add documentation for Exec method

Adds documentation for the Exec method in the DockerContainer struct.
This method executes a command in the current container and
returns the exit status of the executed command, an io.Reader containing
the combined stdout and stderr, and any encountered error.
The documentation emphasizes that reading directly from the io.Reader
may result in unexpected bytes due to custom stream multiplexing headers.
It recommends using stdcopy.StdCopy() from the Docker API to properly
separate the stdout and stderr.

See: https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/503

docker.go

@@ -478,6 +478,13 @@ func (c *DockerContainer) NetworkAliases(ctx context.Context) (map[string][]stri
 	return a, nil
 }
 
+// Exec executes a command in the current container.
+// It returns the exit status of the executed command, an `io.Reader` containing the combined
+// stdout and stderr, and any encountered error. Note that reading directly from the `io.Reader`
+// may result in unexpected bytes due to custom stream multiplexing headers.
+// To separate the stdout and stderr from `io.Reader` and interpret these headers properly,
+// `stdcopy.StdCopy()` from the Docker API should be used.
+// More information about this can be found at: https://godoc.org/github.com/docker/docker/pkg/stdcopy
 func (c *DockerContainer) Exec(ctx context.Context, cmd []string, options ...tcexec.ProcessOption) (int, io.Reader, error) {
 	cli := c.provider.client