Add support to configure Initial Image from global configuration

[leelaprasadv]

Hi @jonhermansen,

Recently Docker announced download rate limit on anonymous docker image downloads and docker-custom-build-environment plugin internally uses an alpine 3.6 call it an initial image/initImage (hard coded inside plugin). This image is downloaded for most job runs.

Here is a PR to make this image configurable in Jenkins Global Configuration under a new section Build In Docker Configuration. This will enable/support configuring the image from local repositories instead of pointing it to docker hub.

Change log:

• Added a new section Build In Docker Configuration in global configuration.
• Added a new text field Init Image to allow the Initial image used by the plugin to be configurable (global.jelly).
• Default the initImage to alpine:3.6.
• Form validations for new field in configuration.
• Help docs for new field.

Preview of the changes:
Configuration preview:

Note: Value defaults to alpine:3.6

Form validation:

~ Prasad.