SAIL_FILE environment variable prevents using docker-compose.override.yml

[ribeirobreno]

Fixes #354 covering the use-cases presented in the docs

• Changed the variable name from SAIL_FILE to SAIL_FILES to better reflect the feature as it is now
• The SAIL_FILES variable accepts multiple paths separated by :, similar to the PATH variable
• Leaving the SAIL_FILES variable empty (or undefined) will not add a -f to the docker-compose command to allow for the default behavior to happen
• Each of the paths is validated if it is a real file before being added to the command:
  • it assumes the path rules from Docker Compose are already taken into account in the configuration
  • it will stop execution early and inform which file didn't pass validation, if any
  • the order of files is preserved as the docs say it does matter

[ribeirobreno]

Examples:

vendor/bin/sail up # Docker compose default files: docker-compose.yml and docker-compose.override.yml
SAIL_FILES="docker-compose.yml:docker-compose.override.yml" vendor/bin/sail up
SAIL_FILES="docker-compose.yml:docker-compose.override.yml:non-existent.yml" vendor/bin/sail up
SAIL_FILES="docker-compose.otherenv.yml" vendor/bin/sail up
SAIL_FILES="non-existent.yml" vendor/bin/sail up

[taylorotwell]

I feel like I have never merged a single PR into Sail that didn't break something, and the logic just becomes more and more complex.

[ribeirobreno]

Shell script is weird, I hope this breaks less than average. hahaha

[curunoir]

Thanks for this improvement