Add namespace option to builder configuration to prevent credential conflicts

[joaoqalves]

Context

When users change Docker Hub (or another registry) accounts in deploy.yml after a successful deploy, kamal deploy fails with authorization errors. This occurs because
Docker buildx builders cache registry credentials, and when the builder name remains unchanged (e.g., "kamal-local-docker-container"), Docker reuses the existing builder with credentials from the previous account.

The buildx builder attempts to push using the new account credentials but still references cached layers from the old account, resulting in "insufficient_scope: authorization failed" errors. Users must manually run kamal build remove to clear the old builder before deploying with a new account.

Solution

We introduce a new optional namespace configuration option under the builder section that allows users to prefix builder names with a custom namespace. This enables users to isolate builders when switching Docker Hub accounts or when running multiple services on the same deployment host, preventing credential conflicts.

This approach was chosen over automatically including the service name in builder names because:

1. Backward compatibility: Existing deployments continue to work without any changes. The namespace option is opt-in, so there are no breaking changes for current users.
2. Flexibility: Users can choose their own namespace strategy. Some may want to namespace by project, by team, by environment, or by Docker Hub account. Automatically using the service name would be too rigid and might not match user needs.
3. Explicit control: Making it a configuration option makes the intent clear and gives users control over when and how to namespace their builders. Automatic service name inclusion could cause confusion if users don't understand why their builder names changed.
4. Opt-in migration: Users experiencing the credential conflict issue can add the namespace option to their deploy.yml and run kamal build remove to migrate, without affecting other users who aren't experiencing the problem.

Usage

builder:
  arch: amd64
  namespace: my-project

Fixes: #1383