Set proper hostname on containers startup

[pyadvichuk]

From my investigation currently there is no way to set hostname defined in config_db.json to containers in automatic mode.
hostname inside container can be updated in case restarting the container (by docker rm -f) however we're runing this only in case hwsku change. So, sudo config reload -y wil not affect hostname inside containers.
Need to pay an attention that some containers (like dhcp_relay) strongly required to have correct hostname as its use it for services runing inside.

If I'm wrong - please point me to the correct way on how to set hostname for containers and simple decline this PR.

- What I did
Set hostname inside each container from <docker-name>.service

- How I did it
Extended existing post-run hook postStartAction

- How to verify it

• manually change 'hostname' in config_db.json
• run sudo config reload -y
• check hostname inside any container

- Description for the changelog
Set proper hostname on containers startup

@lguohan @jleveque: please review this

[qiluo-msft]

We already merged a similar fix #2773
Could you compare both solutions?

[pyadvichuk]

We already merged a similar fix #2773
Could you compare both solutions?

Actually, we cat revert that one as it works only for snmp container. but this will fix hostname for all the existing conyainers.

[qiluo-msft]

I found some new options from docker
https://stackoverflow.com/questions/43031100/when-to-use-hostname-in-docker
Could you have a try?

[pyadvichuk]

I found some new options from docker
https://stackoverflow.com/questions/43031100/when-to-use-hostname-in-docker
Could you have a try?

using this approach we can change hostname only in case hwsku change as I mentioned before.

# ----- docker_image_ctl.j2

start() {
................................
    # Obtain our HWSKU as we will mount directories with these names in each docker
    HWSKU=`sonic-cfggen -d -v 'DEVICE_METADATA["localhost"]["hwsku"]'`
................................
        DOCKERMOUNT=`getMountPoint "$DOCKERCHECK"` #<<<<  grep existing HWSKU from docker inspect
................................
        if [ x"$DOCKERMOUNT" == x"$HWSKU" ]; then
            echo "Starting existing {{docker_container_name}} container with HWSKU $HWSKU"
            preStartAction
            docker start {{docker_container_name}}
            postStartAction
            exit $?

[qiluo-msft]

docker exec -i {{docker_container_name}} bash -c "hostname $HOSTNAME" [](start = 4, length = 69)

It is possible that the early started processes see the original hostname.

[pyadvichuk]

i've checked this using 'dhcp_relay' testcase which is required the hostname to pre-set to a known state and it's working. Anyway, it's better than the existing state as we'll keep hostname to be in sync w/ base system.
In addition - as for my understanding, there is no way to set some specific hostname while starting the container. the only way to set a proper hostname for all processes - remove the previous instance of a container each time.. but this could bring timing issue.

[qiluo-msft]

This is the comparison between your PR and #2773. That one has limited scope on only one container, and don't allow early early processes get the original hostname, because the processes order are controlled by start.sh.

This proposed PR is meant to be general, but has this disadvantage. So it is not qualified for a complete solution.

If you require any fix on specific container, you may follow that PR and fix in the same way.