Expose docker tag information using the API

[rdwebdesign]

What does this PR aim to accomplish?

Docker tag information is available since the introduction of versions file.
The docker info is shown by pihole -v and the web interface footer, but it was never added to the versions API endpoint.

How does this PR accomplish the above?

Adding the information to the API (only inside containers).

The new API response will look like this:

{
  "core_update": false,
  "web_update": false,
  "FTL_update": false,
  "docker_update": false,
  "core_current": "v5.16.2",
  "web_current": "v5.19",
  "FTL_current": "v5.22",
  "docker_current": "2023.03.1",
  "core_latest": "v5.16.2",
  "web_latest": "v5.19",
  "FTL_latest": "v5.22",
  "docker_latest": "2023.03.1",
  "core_branch": "master",
  "web_branch": "master",
  "FTL_branch": "master"
}

Link documentation PRs if any are needed to support this PR:

none

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[yubiuser]

Code looks fine. Could you add a sample output from one of your containers as comment or within the PR description?

[rdwebdesign]

PR description updated.