Add documentation for DOCKER_STRATEGY

What does this MR do?

Add documentation about DOCKER_STRATEGY which allows the user to choose between the life cycle of the container.

Related issues

gitlab-org/gitlab-ee!7553

gitlab-runner!1024 (closed)

gitlab-runner#3605

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails gitlab-org/gitlab-ee!7553

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards