Add .dockerignore to stop host obj/bin leaking into CI image builds

Dockerfile.ci COPYs full project directories after restoring inside
the container. Without a .dockerignore, the host's obj/bin (built
with a different local SDK than the container's) got copied over the
container's fresh restore output, corrupting project.assets.json and
crashing ResolvePackageAssets with a NullReferenceException on publish.

.dockerignore

@@ -0,0 +1,7 @@
+**/bin/
+**/obj/
+**/node_modules/
+**/dist/
+.git/
+.husky/
+docs/