Use official ERDDAP image as base image, substitue env vars in datasets.xml #99
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Use the official ERDDAP Docker image as the base image for this image:
https://hub.docker.com/r/erddap/erddap
Advise users to use the official image directly if they're not using experimental features like datasets.d.
Also support environment variable substitution in datasets.d using envsubst.
Fixes #90, fixes #98