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Use new Dockerfile template, with images from hex.pm

This template is based on the Dockerfile that nowadays is generated with
mix phx.gen.release, and make use of tagged images with well known
versions, rather a rolling tag.
Elixir and OTP version are explicitly set, and should be manually
updated.

Signed-off-by: Davide Bettio <davide.bettio@secomind.com>