chore: Add .dockerignore to fix Docker build out-of-space error

Excludes target/, coverage/, flawd-report/, and .git/ from the Docker
build context to prevent 'no space left on device' during COPY.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: fohara

.dockerignore

@@ -0,0 +1,7 @@
+target/
+coverage/
+flawd-report/
+.git/
+.DS_Store
+*.swp
+*.swo