Add config from environment and config path flag

[jsoriano]

Thinking about adding the TLS flags for #711, I found that it could be handy, specially for the containerized registry, to allow to include these settings using environment variables (see #746).

So for example to add certificates to a containerized registry, one needs to mount the certificates as volumes, and add the environment variables, what is a common practice. Using flags only, the command line needs to be modified, what can override other defaults as the default address or -disable-package-validation, included in current distribution images.

This change includes:

• Some documentation notes about how the registry can be configured.
• A new flag -config to select the configuration file.
• Flags can be provided using environment variables. Flags have precedence over environment variables.

Related to #711.

[elasticmachine]

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• Start Time: 2021-10-12T11:37:37.475+0000

• Duration: 5 min 29 sec

• Commit: 2c79acf

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[mtojek]

I like this idea, but let's document in the repository's README.

[jsoriano]

I like this idea, but let's document in the repository's README.

It is already mentioned in the paragraphs added to the README, should I add something else there?

[mtojek]

I missed this while reviewing. Maybe add also a sample one liner showing how it maps (-dry-run vs EPR_DRY_RUN).