Improve docker UX

[josecelano]

Relates to: #351

• Improve the specific documentation for docker: https://github.com/torrust/torrust-tracker/tree/develop/docker
• Write a command to generate the default storage files before running the docker image. The installation process is here, but we need to write a gist or something like that so you do not need to clone the repo.
• We should make the same change we did for the backend. The backend allows changing the default config file location with an env var: TORRUST_IDX_BACK_CONFIG_PATH. This way could create the config file in the storage folder without injecting the env var (although env va also works).
• Improve the error message when the config file is not found. It should say something like: "Can load the configuration file from ./config.toml"