Telegraf docker entrypoint script change needed to support deployment from compose script

Referencing #724 and #543 I was still having problems with a specific line in the [entrypoint script](https://github.com/influxdata/influxdata-docker/blob/0f0bdbbb22b44e9a5953b0f733d6992ade4d7c55/telegraf/1.30/entrypoint.sh#L21)

The line currently reads:

`extra_groups="$(id -Gn || true)"`

This script doesn't honor UID; rather, if the UID is 0 (root) it just executes as the 'telegraf' user. Unfortunately, if you are root, that id will get the groups associated with root, not telegraf. I think it needs to be changed to:

`extra_groups="$(id -Gn telegraf || true)"`

With this change, I can specify a service in a compose file:

```
services:
telegraf:
image: mystack/telegraf
container_name: telegraf
volumes:
- type: bind
source: /etc/telegraf
target: /etc/telegraf
read_only: true
- type: bind
source: /var/run/docker.sock
target: /var/run/docker.sock
build:
context: telegraf
```

I made this change with a custom entrypoint to test it:

```
FROM docker.io/telegraf
RUN groupadd -r docker -g 122
RUN usermod -aG docker telegraf
COPY custom-entrypoint.sh /custom-entrypoint.sh
RUN chmod +x /custom-entrypoint.sh
ENTRYPOINT ["/custom-entrypoint.sh"]
CMD ["telegraf"]
```
and that makes it work.

I think there's about a million ways to solve this problem. I think, though, that the structure of the script really intends to just exec if the uid is not 0, and if the uid is 0 it intends to force you to the 'telegraf' user - that's a strategy and it's fine and reasonably secure, but that `id -Gn` is going to check the extra groups of user 0, come up with only "root" then promptly remove that group.

If anybody agrees, please respond - I'll happily submit a pull request for this. Or, if you've got another way to pair docker compose + docker build and make this work, that's fine too. Me having to create a custom entrypoint doesn't seem like the right solution unless I'm the only person still having this problem.

Start with telegraf/1.30/entrypoint.sh at line 21 and inspect how its group lookup behaves when the container starts as root before switching to telegraf. Reproduce the Docker Compose bind-mount scenario from the issue and confirm that the entrypoint preserves the telegraf user's extra group access.

Written by the indexing model from the issue text.