Dockerfile: create zig cache via run.sh, not zig test (#70)

Three recent commits [1][2][3] tried to speed up running user solutions
in production, but weren't sufficient. Continue trying, by running
`zig test` indirectly.

Extra advantage: we have a single source of truth for `zig test`
options, which helps if we want to change them later.

Disadvantage: `docker build` no longer fails if this line produces a
failing test. That's why I didn't do it this way originally.

Refs: #63

[1] 82b56af, 2023-08-23, ".dockerignore, Dockerfile: add a zig cache"
[2] b8b9d02, 2023-08-27, "Dockerfile, bin(run-tests), tests: set USER"
[3] 3f2c0c2, 2023-08-27, "Dockerfile: create zig cache later"

Author: ee7

Dockerfile

@@ -34,6 +34,6 @@ COPY --chown=ziggy:ziggroup bin/run.sh bin/run.sh
 # Initialize a zig cache
 COPY --chown=ziggy:ziggroup tests/example-success/example_success.zig init-zig-cache/
 COPY --chown=ziggy:ziggroup tests/example-success/test_example_success.zig init-zig-cache/
-RUN zig test init-zig-cache/test_example_success.zig \
+RUN bin/run.sh example-success init-zig-cache init-zig-cache \
     && rm -rf init-zig-cache/
 ENTRYPOINT ["/opt/test-runner/bin/run.sh"]