Dockerfile: create zig cache later (#69)

Two recent commits [1][2] tried to speed up running user solutions in
production, but weren't sufficient.

Make the `ziggy` user, rather than the user in the earlier stage, create
the cache. This is a worse Dockerfile pattern [3], but it might help Zig
to use the cache.

A zig cache directory like `~/.cache/zig` contains an `h` subdirectory,
with `.txt` files like:

    0
    18137 11111111 1692020106000000000 0d5def4e330aca188b4480e29786a6c0 1 c.zig
    18137 11111111 1692020106000000000 0d5def4e330aca188b4480e29786a6c0 1 c.zig
    9163 22222222 1692020106000000000 514b9d62f95db9ea86d2cd5008c5d946 1 std/std.zig
    9163 22222222 1692020106000000000 514b9d62f95db9ea86d2cd5008c5d946 1 std/std.zig
    22609 33333333 1692020106000000000 0538267a6cdadf6d7dadf7693fce1bed 1 std/start.zig
    [...]

It looks like the format is [4]:

    manifest header (0), then rows of:

    {size} {inode} {mtime_ns}, {crypto.auth.siphash.SipHash128 hash} {prefix} {sub_path}

where the prefixes are commented as:

    /// A set of strings such as the zig library directory or project source root, which
    /// are stripped from the file paths before putting into the cache. They
    /// are replaced with single-character indicators. This is not to save
    /// space but to eliminate absolute file paths. This improves portability
    /// and usefulness of the cache for advanced use cases.

but maybe the cache didn't allow what we were doing before this
commit: creating it with one user at some location, then moving it to
another location to be used by a different user.

If this commit still isn't sufficient, we could try updating Zig.
Exercism currently uses the Zig 0.11.0 release, which doesn't include a
recent commit for the zig cache system [5].

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] https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#use-multi-stage-builds
[4] https://github.com/ziglang/zig/blob/0.11.0/lib/std/Build/Cache.zig#L840-L851
[5] ziglang/zig@020105d0dde6
    2023-08-20, "Cache: Fix findPrefix when paths are slightly out of the ordinary"

Author: ee7

Dockerfile

@@ -17,14 +17,6 @@ RUN tar -xvf ${RELEASE}.tar.xz \
     && rm -rf /tmp/${RELEASE}/lib/libc/include/any-windows-any \
     && 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
@@ -34,10 +26,14 @@ RUN apk add --no-cache jq
 RUN addgroup ziggroup \
     && adduser --disabled-password --gecos ziggy --ingroup ziggroup ziggy
 COPY --from=builder --chown=ziggy:ziggroup /opt/zig/ /opt/zig/
-COPY --from=builder --chown=ziggy:ziggroup /root/.cache/zig/ /home/ziggy/.cache/zig/
 ENV PATH=$PATH:/opt/zig
 
 USER ziggy:ziggroup
 WORKDIR /opt/test-runner
 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 \
+    && rm -rf init-zig-cache/
 ENTRYPOINT ["/opt/test-runner/bin/run.sh"]