Improve Docker detection for better Chromium execution

[yhatt]

Marp CLI official Docker image had defined IS_DOCKER env to tweak internal settings into options that are suited for Docker. However, this approach does not assume an general Docker image. So setting up Marp CLI in a homemade image may become difficult, as pointed out at marp-team/marp#165.

This PR will improve Docker detection in Marp CLI.

• Uses is-docker module to detect whether Marp CLI is running within a Docker image, and apply better Chromium options when executing the browser. This module has already used by is-wsl internally.
• Make generalize a setting of official Docker image. We no longer use IS_DOCKER env, and use a standard way CHROME_PATH env to set a specific path to Chromium.
• Some specific restrictions for the official image like disabled GUI option (--preview) will set by detecting MARP_USER env, that is assumed for the official image. They restrictions are not applied to a general image. (GUI app within Docker may render to any X server by right settings)