Exclude _image_sources directory from Docker context to reduce build transfer size

[Copilot]

The _image_sources directory contains 119MB of source design files (SVGs, PNGs, XCF files) that are not needed during Docker builds. This directory was causing unnecessary data transfer during Docker context creation.

Changes Made

Added _image_sources to .dockerignore to exclude it from Docker build context.

Impact

• Docker context size reduction: 549.09MB → 197.41kB (99.96% reduction)
• Significantly faster Docker builds due to reduced context transfer time
• No functional impact on the application

Verification

• ✅ Site builds successfully with npm run dist and npm run 11ty
• ✅ Site serves correctly with npm run serve
• ✅ Directory already excluded from Eleventy builds via .eleventyignore
• ✅ No direct code references found (only used in GitHub workflow triggers)
• ✅ Contains source materials not required for runtime builds

The _image_sources directory appears to contain design source files used for creating final assets, similar to how source code gets compiled into production builds. These files don't need to be present in the Docker container during builds or runtime.

Fixes #223.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.