Ensure no manual input in 'dnf' install

-y flag in dnf, Automatic yes to prompts. Assume "yes" as answer to all prompts and run non-interactively. If an undesirable situation, such as changing a held package or removing an essential package, occurs then dnf will abort.

Risk Level: medium
Platform: Docker
Spectral Rule ID: DOCKR025


use -y flag in each use of dnf install

FROM fedora:27
RUN set -uex && \
dnf config-manager --set-enabled docker-ce-test && \
- dnf install docker-ce && \
+ dnf install -y docker-ce && \
dnf clean all

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