.dockerignore, Dockerfile: add a zig cache (#59)

It took Exercism much longer to run the tests for a Zig exercise than it
should. This was because Zig had never been run in the container before
test time, and there was no compilation cache in the image.

Let's try an initial simple approach: run `zig test` once
(for `tests/example-success`) and copy the resulting zig cache into the
image. This seems like a 2.2x speedup when later testing a typical
exercise, at the cost of adding a 41 MB zig cache to the image:

    33 MB  /root/.cache/zig/z/
     8 MB  /root/.cache/zig/o/
    22 kB  /root/.cache/zig/h/

Later, I believe we can speed up further by caching the result of
compiling more functions from e.g. std.testing.

Closes: #28

Author: ee7

.dockerignore

@@ -11,4 +11,3 @@ Dockerfile
 bin/run-in-docker.sh
 bin/run-tests-in-docker.sh
 bin/run-tests.sh
-tests/

Dockerfile

@@ -16,15 +16,24 @@ RUN tar -xvf ${RELEASE}.tar.xz \
     && rm -rf /tmp/${RELEASE}/doc \
     && mv /tmp/${RELEASE} /opt/zig
 
+# Initialize a zig cache
+ENV PATH=$PATH:/opt/zig
+WORKDIR /opt/test-runner
+COPY tests/example-success/example_success.zig init-zig-cache/
+COPY tests/example-success/test_example_success.zig init-zig-cache/
+RUN zig test init-zig-cache/test_example_success.zig \
+    && rm -rf init-zig-cache/
+
 FROM ${REPO}:${IMAGE} AS runner
 
 # install packages required to run the tests
 # hadolint ignore=DL3018
 RUN apk add --no-cache bash jq
 
 COPY --from=builder /opt/zig/ /opt/zig/
+COPY --from=builder /root/.cache/zig/ /root/.cache/zig/
 ENV PATH=$PATH:/opt/zig
 
 WORKDIR /opt/test-runner
-COPY . .
+COPY bin/run.sh bin/run.sh
 ENTRYPOINT ["/opt/test-runner/bin/run.sh"]