Add Dockerfile

[JenAks]

Summary

• Adds a multi-stage Dockerfile for the Maven/Java project
• Build stage uses Maven 3.9 + JDK 17 to run mvn install
• Runtime stage uses a slim JRE 17 image

Test plan

• Verify Seal Agent inserts Seal CLI step in correct position relative to mvn install

🤖 Generated with Claude Code

[desruisseaux]

Thanks for this submission, we appreciate. But this pull request is based on the 1.2 tag (the current development version is 1.7-SNAPSHOT) and contains changes in 159 files, which is probably much more than intended. It would need to be rebased on the current master.

An issue may be that this pull request expect a Maven build, which Apache SIS temporarily switched to Graddle. We will switch back to Maven 4 when it will be ready. But in the meantime, this pull request may need to be put on ice.