Add Remote Cache Support

[spboyer]

Description

Support remote/shared cache for faster builds in CI/CD environments.

Current State

Only local filesystem caching available.

Proposed Implementation

• Support registry-based cache storage
• S3/Azure Blob/GCS cache backends
• Cache import/export commands
• Configurable cache keys and scope

Use Cases

• Shared cache across CI/CD jobs
• Team-wide build acceleration
• Multi-runner environments

Priority

High - Critical for CI/CD performance.

Example

pycontainer build --cache-to type=registry,ref=myregistry.io/cache
pycontainer build --cache-from type=registry,ref=myregistry.io/cache

Parent: #49