fix(docker): exclude .venv from Docker build context

Without this, the server's local .venv gets copied into the image by
the final COPY . . step in Dockerfile.eval-base, overwriting the
freshly-created uv venv. uv then sees those packages as already
installed and skips them — but they may be missing or built for the
wrong environment, causing ModuleNotFoundError at runtime.

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

.dockerignore

@@ -12,6 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Python virtual environments — never copy into Docker images
+.venv
+venv
+env/
+
 # Misc
 .git
 tmp