Introduce a separate base docker image for eclair runtime

[darwin]

The copy&pasted Dockerfile content must be kept up-to-date and that is fragile. For best compatibility the image should be based on openjdk:8u171-jdk-alpine (see https://github.com/ACINQ/eclair/blob/master/Dockerfile#L1) and our own stuff should be added on top.

See ACINQ/eclair#1041.