feat: support includes an excludes (#12)

[schroedan]

Closes #12

[Copilot]

Pull request overview

Adds opt-in filtering for which _FILE-suffixed environment variables fileenv will resolve, to support common cases where a base image sets unrelated *_FILE variables (e.g., SSL_CERT_FILE).

Changes:

• Introduces FILEENV_EXCLUDE and FILEENV_INCLUDE configuration (mutually exclusive) and applies filtering during environment scan.
• Adds a small parser for comma-separated include/exclude lists.
• Documents the new configuration options and an example use case in the README.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
README.md	Documents FILEENV_EXCLUDE / FILEENV_INCLUDE, their mutual exclusivity, and an example excluding SSL_CERT_FILE.
main.go	Implements include/exclude config parsing and filtering during _FILE variable resolution.

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