Jobs should be better isolated to avoid interference with other `image` or...

What does this MR do?

While templates in gitlab-ci ae a good improvement, the definition of the Docker.gitlab-ci.yml interfere with other eventual image or before_script statements. In my experience:

include:
  - template: 'Docker.gitlab-ci.yml'

image: debian:stable-slim

before_script:
  - apt update -y -q && apt install openssh -y -q

myjob:
  - echo hello

raises and error since .gitlab-cy.yml image overrides the included image statement the when the meshed before_script script is executed the docker login command fail since the image isn't the docker one.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team