Env var behaviour for cache removal.

[born2discover]

This fixes #6 and introduces a new environment variable "PLATFORM" which when defined and set to "DOKKU" would force the buildback not to perform the cache deletion command.
Otherwise if the env variable is not set, or set to "HEROKU" then that command will be performed.

For Dokku users it means adding an additional export statement in their .env file.

Example contents of .env file:

# Can be either DOKKU or any other string different from HEROKU.
export PLATFORM="DOKKU"
export BUILDPACK_URL=https://github.com/born2discover/heroku-buildpack-pelican

[julbrs]

+1 for this PR. Working very well on my dokku+pelican apps,

Thanks for the work,

[justinmayer]

Any thoughts on this PR, @kylef?

[chadselph]

Looks like this never got merged :( and @born2discover removed their fork

[born2discover]

Looks like this never got merged :( and @born2discover removed their fork

@chadselph : As I am not using this build-pack myself anymore I indeed have recently removed my fork. However the changes I proposed are still available on Github, just check the "Files changed" tab, so the proposed feature can still be implemented.