Add instructions for local testing with s390x

DEVELOPMENT.md

@@ -9,3 +9,58 @@ The following command can be used to generate the `compile_commands.json` file:
 ```
 BAZEL_BUILD_OPTION_LIST="--define=engine=multi" ./tools/gen_compilation_database.py --include_all //test/... //:lib
 ```
+
+## Testing s390x locally
+
+The s390x build-tools image used in our CI environment does not contain all the tools necessary to build test wasm modules. First, build those wasm modules in your native environment:
+
+```bash
+bazel build --verbose_failures --test_output=errors --config=clang --config=hermetic-llvm -c opt $(bazel query 'kind(was.*_rust_binary, //test/test_data/...)') $(bazel query 'kind(_optimized_wasm_cc_binary, //test/test_data/...)')
+
+# Copy the built artifacts to the test_data dir.
+cp bazel-bin/test/test_data/*.wasm test/test_data/
+
+# Rewrite test/test_data/BUILD to use the pre-built wasm module artifacts
+echo "package(default_visibility = [\"//visibility:public\"])" > test/test_data/BUILD
+for i in $(cd test/test_data && ls -1 *.wasm | sed 's/\.wasm$//g'); do \
+  echo "filegroup(name = \"$i\", srcs = [\"$i.wasm\"])" >> test/test_data/BUILD; \
+done
+# Rewrite test/BUILD to use the pre-built artifacts
+sed 's/\.wasm//g' test/BUILD > test/BUILD.tmp && mv test/BUILD.tmp test/BUILD
+```
+
+s390x tests can be run locally using the build-tools docker images published in
+this repo. An s390x QEMU emulator is required to be installed ([example
+instructions](https://docs.gitlab.com/omnibus/development/s390x/#vm-provisioning)).
+The following command will start an interactive session in an emulated s390x
+environment with bazel installed. The command mounts $HOME inside the container
+and assumes proxy-wasm-cpp-host is somewhere under $HOME.
+
+```bash
+docker run --env HOME=$HOME --env USER=$(id -un) --volume "$HOME:$HOME" --workdir $(pwd) --user $(id -u):$(id -g) --platform linux/s390x --interactive -t --env PS1="docker (s390x):" ghcr.io/proxy-wasm/build-tools:ubuntu-24.04-bazel-7.7.1 bash
+
+# Set a useful prompt to remind yourself this shell is in s390x:
+export PS1='docker@s390x:\[\033[01;34m\]\w\[\033[00m\] )> '
+```
+
+Inside this shell, you may now call out to bazel to run any build or test
+command using the s390x environment. Note that `--config=hermetic-llvm` is not
+supported for s390x.
+
+```bash
+bazel test --verbose_failures --test_output=errors --define engine=wasmtime --config=clang --test_timeout=1800 //test:runtime_test --test_filter=\*PassingValue\*
+```
+
+
+### Troubleshooting
+
+If you experience errors relating to GCC_DUMMY_TOOLCHAIN, re-check that your
+test/test_data/BUILD file contains only `filegroup()` rules pointing at
+pre-build wasm modules.
+
+If you get a "Required file not available" error message when executing a
+toolchain with your host architecture in the file path, it is likely due to
+bazel re-using a cache from your host environment. Please run `bazel clean
+--expunge` and `bazel shutdown` in the docker container to clear bazel's cache
+analysis cache.
+