Add S3 Docker build cache support

[WVerlaek]

Description

Add support for S3-backed Docker build layer caching using BuildKit's type=s3 cache backend. This enables faster Docker builds by caching intermediate layers in S3.

New environment variables:

• LEEWAY_DOCKER_S3_CACHE_BUCKET - S3 bucket name (required)
• LEEWAY_DOCKER_S3_CACHE_REGION - AWS region (required)
• LEEWAY_DOCKER_S3_CACHE_PREFIX - Optional prefix for cache keys
• LEEWAY_DOCKER_S3_CACHE_MODE - Cache mode: min or max (default: max)
• LEEWAY_DOCKER_S3_CACHE_ENDPOINT - Custom S3 endpoint for S3-compatible storage

New CLI flags:

• --docker-s3-cache-bucket
• --docker-s3-cache-region
• --docker-s3-cache-prefix
• --docker-s3-cache-mode
• --docker-s3-cache-endpoint

When S3 cache is enabled, leeway automatically uses docker buildx build with --cache-from and --cache-to flags pointing to the configured S3 bucket.

Related Issue(s)

Fixes CORE-

How to test

# Via environment variables
export LEEWAY_DOCKER_S3_CACHE_BUCKET=my-cache-bucket
export LEEWAY_DOCKER_S3_CACHE_REGION=us-east-1
export LEEWAY_DOCKER_S3_CACHE_PREFIX=docker-cache/
leeway build :my-docker-package

# Via CLI flags
leeway build \
  --docker-s3-cache-bucket=my-cache-bucket \
  --docker-s3-cache-region=us-east-1 \
  --docker-s3-cache-prefix=docker-cache/ \
  :my-docker-package

# Verify the build command includes cache flags
leeway build -v :my-docker-package 2>&1 | grep cache

Requirements:

• Docker Buildx
• AWS credentials configured (via environment variables, IAM role, or AWS config file)
• S3 bucket with appropriate permissions