Set docker_image to the base image in every task.toml
Adds docker_image = "docker.io/savatar101/env-data-agent-train:base" under [environment] in all 250 task.toml files: the image every task's Dockerfile already builds FROM, which is all those Dockerfiles do besides WORKDIR /workdir. Harbor then uses the prebuilt image directly instead of building one per task, and the test set runs on HF sandboxes, which need a prebuilt image. Same change as the train set (https://huggingface.co/datasets/FineEnvs/SmolDataEnvs-harbor-train/discussions/2), except for the image reference: this uses the :base tag rather than the digest. HF Jobs uses the image as a label, capped at 100 characters, and docker.io/savatar101/env-data-agent-train@sha256:... is 113, so a digest pin makes every HF sandbox fail with Too big: expected string to have <=100 characters at labels.name.