NX_CONTAINER_SHORT_SHA_LENGTH documentation in container-metadata

[monument-bmann]

@jscutlery/semver generates a short git hash of 8 characters in length. Where @nx-tools/container-metadata default is 7.

I had to dig into the code to find this process.env.NX_CONTAINER_SHORT_SHA_LENGTH, to figure out where our deployment scripts were off.

Suggest this and any other env variables be documented.