Add Dockerfile Import/Migration Tool

[spboyer]

Description

Tool to import/migrate existing Dockerfiles to pycontainer-build configuration.

Proposed Implementation

• Parse Dockerfile syntax
• Convert to pyproject.toml config
• Generate equivalent pycontainer settings
• Provide migration guide and warnings

Use Cases

• Migrating from Docker-based builds
• Evaluating pycontainer-build
• Reducing adoption friction

Example

pycontainer import Dockerfile --output pyproject.toml

Priority

Medium - Eases migration path.

Parent: #53