Add builder.additional_contexts configuration to natively pass additional build contexts

[crydafan]

Problem

While migrating from Docker Compose to Kamal in production, one of our Dockerfiles copies scripts from a context inside the app context:

COPY --from=scripts . .

Docker Compose has a native option to declare these extra build contexts via additional_contexts:

additional_contexts:
  scripts: ../../scripts

Kamal had no equivalent, so the build broke during migration — there was no way to pass --build-context to Buildx from the deploy config.

Fix

Kamal now supports declaring additional build contexts under builder.additional_contexts in deploy.yml:

builder:
  context: "../../apps/dashboard"
  dockerfile: "../../infra/docker/dashboard.Dockerfile"
  additional_contexts:
    scripts: "../../scripts"
    assets: "../../assets"

Each entry is passed through to Buildx as --build-context <name>=<value> and can be referenced in the Dockerfile via COPY --from=<name>.

Implementation

• Config parsing (lib/kamal/configuration/builder.rb): added an additional_contexts reader that returns the configured hash (or {}).
• Validation (lib/kamal/configuration/validator.rb): additional_contexts is treated as a free-form Hash (like args/options) since its keys are user-defined; the option is also documented in docs/builder.yml, which doubles as the schema example.
• Command building (lib/kamal/commands/builder/base.rb): added a build_additional_contexts method that formats each key/value pair using the existing argumentize helper and appends the result to build_options. All drivers (local, remote, hybrid, cloud) inherit from Base, so they all pick this up.
• Tests: added coverage for config parsing (default + configured) and for the generated --build-context flags in both build_options and the full docker buildx build command.