fix: Docker warning when using frapsoft/openssl on arm64 system

Resolved a warning from Docker that would get printed when running the generate-test-keys Makefile target on an arm64 system. The warning from Docker alerts you that the image for frapsoft/openssl is for amd64, which doesn't match the system you're on (when using an arm64-based OS). To resolve the warning, you have to either use an image that matches the system architecture, or explicitly state the platform using the --platform flag. In this case, there is only an amd64 version of frapsoft/openssl, so I added the explicit flag to resolve the warning. I also added a TODO for us to find an arm64 alternative in the future (although the amd64 version of frapsoft/openssl does work fine on arm64 macOS currently as it seems that Docker makes use of QEMU to support emulation of non-native architectures).

Signed-off-by: Derek Trider <Derek.Trider@securekey.com>

Makefile

@@ -88,9 +88,12 @@ vc-test-suite: clean
 bbs-interop-test:
  @scripts/check_bbs_interop.sh
 
+# TODO (#3373): frapsoft/openssl only has an amd64 version. While this does work when using Docker on arm64-based macOS
+# currently thanks to the automatic built-in QEMU emulation, it would be better to use a
+# native arm64 version.
 generate-test-keys: clean
  @mkdir -p -p test/bdd/fixtures/keys/tls
- @docker run -i --rm \
+ @docker run -i --platform linux/amd64 --rm \
  -v $(abspath .):/opt/go/src/$(PROJECT_ROOT) \
  --entrypoint "/opt/go/src/$(PROJECT_ROOT)/scripts/generate_test_keys.sh" \
  frapsoft/openssl