[EPIC] Provide support for websites stored in git

[ieugen]

Hi,
(summary of discussions with @alekcz)

pcp can be used to deploy sites from git.
This issue summarizes some ideas / solutions on how to achieve that.

Current proposal:

pcp should wrap/rely on the native git command line for git operations.
This is much better than integrating jgit which can have compatibility issue.

Going forward, git client is assumed to be installed on the host for this feature.

Provide access to private git repositories

There are a few solutions here:

• Use username and pass in git url
• Provide a file that contains the information

See more information:

• https://git-scm.com/docs/gitcredentials
• https://www.baeldung.com/ops/git-configure-credentials

Repo sync

Checking for updates can be done via

• pooling (via a periodic job)
• git webhook notification - when a commit is made the server will notify us

Polling can be done via a cron job that

• checks all repositories every X minutes
• interval is user configurable at deploy
• functionality should already exist (and be usable in a docker image)
  ** https://github.com/muesli/gitomatic
  ** https://github.com/Kriechi/git-monitor
  ** https://github.com/ivanwills/git-workflow-tools/blob/master/lib/App/Git/Workflow/Command/Watch.pm
  ** https://github.com/clj-jgit/clj-jgit (perhaps)
  ** https://gist.github.com/CraZySacX/10138634

Update can be triggered by an API event (webhook)

• more flexible
• more complex

[ieugen]

This functionality can be provided by publishing a docker image on top of the bare bones pcp docker image.

  docker pull pcp:123
  docker pull pcp:123-git-sync