Add socket activation for RHEL based distributions

[seemethere]

Removes the systemd drop-in unit file for socket activation and instead
prefers socket activation by default for both RHEL based and DEBIAN
based distributions.

Socket activation for RHEL based distributions was tested on CentOS 7 and Fedora 28.

Signed-off-by: Eli Uriegas eli.uriegas@docker.com

[andrewhsu]

LGTM

[thaJeztah]

Was there a reason for not using a drop-in file?

Also note that this will be a breaking change for RHEL/CentOS users that currently set the host through daemon.json; try using;

mkdir -p /etc/docker
echo '{"hosts":["tcp://localhost:2375"]}' > /etc/docker/daemon.json

And then restart the docker.service

[andrewhsu]

This will be a breaking change for users who set the host through daemon.json and have installed docker-ce from the nightly channel in the past 13 days since #257 was merged, but this will not be a breaking change in that same way for 18.09 users when this change is ported to 18.09 codeline because there was already a host port set: https://github.com/docker/docker-ce/blob/v18.09.0/components/packaging/systemd/docker.service#L13

[thaJeztah]

but this will not be a breaking change in that same way for 18.09 users when this change is ported to 18.09 codeline because there was already a host port set

In previous releases, the host was only set on .deb packages, and not on RPM packages (because it didn't have socket activation) https://github.com/docker/docker-ce/blob/v18.06.1-ce/components/packaging/rpm/systemd/docker.service#L12

[andrewhsu]

Socket activation was removed from docker engine in RPMs long time ago because of selinux policy incompatibilities, but since then the selinux policies are now compatible with socket activation. @seemethere had tested this PR's changes on a test build fedora and centos RPM packages.

Since we did not have host port set in 18.06.1 RPM packages, I would not recommend backporting this change to the 18.06 codeline.

Whereas since we already set the host port in 18.09.0 RPM packages, I would offer that it is safe to backport this change to the 18.09 codeline for the future 18.09.1 release.

[thaJeztah]

Alright; if we do, we should make sure there's a note in changelog/release-notes with instructions how to resolve the issue if you're upgrading from an older release.

(And we should have a follow-up discussion around how daemon.json and flags should interact; possibly an exception for -H fd:///)