Update README.md

[haydonryan]

I run arch btw... docker build -t bosh-io-release-resource --target tests . fails due to the following error message:

$ docker build -t bosh-io-release-resource --target tests .
DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/

Sending build context to Docker daemon  1.417MB
Step 1/10 : ARG base_image=cgr.dev/chainguard/wolfi-base
Step 2/10 : ARG BUILDPLATFORM
Step 3/10 : FROM --platform=${BUILDPLATFORM} ${base_image} AS tests
failed to parse platform : "" is an invalid OS component of "": OSAndVersion specifier component must match "^([A-Za-z0-9_-]+)(?:\\(([A-Za-z0-9_.-]*)\\))?$": invalid argument
(base)                                                              

Adding required parameter to actually run the test on linux and other os.