Dockerfile: use bats-core
Dockerfile: use bats-core

The bats testing framework we use for integration test is not maintained
since 2015 and was superceded by bats-core [1]. More to say, we were
using an unreleased version and relying on some features of it
(unfortunately I don't remember now what are those features exactly).

As Debian still packages very old version of bats from the old repo,
so let's Use recent bats-core from the new, supposedly better maintained,
github repo.

[1] https://github.com/sstephenson/bats/pull/269

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

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