Fix rootfs dir

[sbomer]

This fixes the rootfs dir for the images added in #754, to be consistent with the existing images.

I looked for a way to extract this into a single variable in the Dockerfile. The only way I found was to use ENV, but that would persist the environment variable into the created image - and I want to avoid polluting the image's environment.

[sbomer]

@mthalman the build is failing during apt-get update with:

W: GPG error: https://apt.kitware.com/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 42D5A192B819C5DA

I believe the kitware signing keys have been updated, but we are still using a cached base image with an older signing key, the one obtained here:

dotnet-buildtools-prereqs-docker/src/ubuntu/20.04/crossdeps/Dockerfile

Line 11 in d2b3b2b

&& wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc | apt-key add - \

How would I go about updating the base images? (As a longer-term solution, it looks like there is a package that will keep signing keys updated - kitware-archive-keyring, mentioned here: https://apt.kitware.com/).

[mthalman]

@sbomer - If you make an arbitrary change to the base Dockerfile, it will trigger it to get rebuilt. Then you can revert your change. It's based off commit SHA, so it should still rebuild even after you revert your change.

[mthalman]

If this got you the build results you're looking for, go ahead and revert the crossdeps Dockerfile change.