added mount to docker-driver config

[tonycox]

related to:
#694
#700

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Documentation added
• Tests updated

[claassistantio]

All committers have signed the CLA.

[tonycox]

As I'm not sufficient good in Go - I ve just proposed this change

[cyriltovena]

Can you explain how to use this ? I think I have an idea but I would like to see what was your plan.

[tonycox]

currently I need to do this trick while deploying my app with docker

LOKI_ID=$(sudo docker plugin inspect -f '{{.Id}}' loki)
sudo rm -rf /var/lib/docker/plugins/$LOKI_ID/rootfs/opt/tool; mkdir -p /var/lib/docker/plugins/$LOKI_ID/rootfs/opt/tool
cp docker/loki-pipeline.yml /var/lib/docker/plugins/$LOKI_ID/rootfs/opt/tool/loki-pipeline.yml

It would be better to specify visible folders for plugin in env configuration scripts. for example workspace folder path of an app.

/var/log is just a placeholder - it can be empty or more general.

[cyriltovena]

how can it be empty ? where will you put your file after ? I think this need a bit of documentation.

[tonycox]

for file on my local env /folder/sub/pipeline.yml
I'll set to the source param by

docker plugin set loki mount.source=/folder/sub

and then I'll set

docker run --log-driver=loki \
    ...
    --log-opt loki-pipeline-stage-file=pipeline.yml

[cyriltovena]

Yeah I think this is good, I would like a bit of documentation on the usage in the PR if you don't mind.

[tonycox]

Ok, I'll take time to generate details in the PR

[cyriltovena]

That would be great to do a step by step on how to mount the config.

• disable the plugin.
• set the mount.
• enable it again.

a new chapter here https://github.com/grafana/loki/blob/master/docs/clients/docker-driver/configuration.md

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[m0rtalis]

Can this be added? I think @mfut did basically all the work in his PR maytech#1