Copy configs.file's and secrets.file's instead of bind-mounting them to
make it possible to use file configs when working with remote docker
hosts (like setting DOCKER_HOST to a ssh address or setting docker
context)

Includes support for config.files and secrets.files as directories.

Note that file.Content as source of secrets is denied elsewhere with the
error "validating docker-compose.yml: secrets.content_secret Additional
property content is not allowed", but it is implemented here in case
this restriction is liften in the future.

Configs and secrets from environment is also handled as plain content
inserted into target file.

implements: docker#11867

If target is relative it should be joined with target base, not
concatenated.