Fix dockerrun.sh to dynamically add CLI build environment variables to the container

[eerhardt]

The current way we pass environment variables to our docker containers leads to errors and duplication. See https://github.com/dotnet/cli/blob/rel/1.0.0/scripts/dockerrun.sh#L115-L134 where we need to keep updating the environment variables that are passed in. For example, dotnet/cli#5274.

Instead, we should make this dynamic so we don't have to keep this list up to date.

One idea I had was to use printenv to get the list of environment variables. Grab all the ones that start with a prefix like CLIBUILD_, and write them out to an env-file. Then pass that file to docker run --env-file. See https://docs.docker.com/engine/reference/commandline/run/#/set-environment-variables--e---env---env-file.

[github-actions]

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