feat: Add platform details to help for snyk container

The snyk container CLI now supports passing an explicit platform value.
This will then test the image with that platform, rather than just the
default (probably amd64).

Author: garethr

help/container.txt

@@ -16,6 +16,9 @@ Options:
   --file=<string> ......................... Include the path to the image's Dockerfile for more detailed advice.
   -h, --help
   --json
+  --platform=<string> ..................... For multi-architecture images, specify the platform to test. Options are:
+                                            [linux/amd64, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x,
+                                            linux/386, linux/arm/v7 orlinux/arm/v6]
   --print-deps ............................ Print the dependency tree before sending it for analysis.
   --project-name=<string> ................. Specify a custom Snyk project name.
   --policy-path=<path> .................... Manually pass a path to a snyk policy file.
@@ -24,6 +27,7 @@ Options:
 Examples:
 
   $ snyk container test alpine
+  $ snyk container test --platform=linux/arm64 debian
   $ snyk container monitor alpine
   $ snyk container test docker-archive:archive.tar
   $ snyk container test oci-archive:archive.tar