add `HEALTHCHECK`instruction in Alpine Dockerfile

[mouhamed]

What does this PR do?

This PR aims to add HEALTHCHECKinstruction in Alpine Dockerfile so that user does not have to systematically add these important instruction in his compose file

Motivation

Best practice! Some users don't know how to healthcheck a traefik instance

Additional Notes

I also added a .dockerignore so that templates are not included in Docker context

[ldez]

Hello,

We preferred to let the user define the healthcheck by itself (in a docker-compose file by example).

The command traefik healthcheck require that:

• the ping must be enable
• the configuration must use traefik.toml (CLI args will not work)

So I have to decline, thanks anyway.

[dduportal]

Additionally, the healthcheck is a docker only instruction not recognized in other container runtimes, neither in Nomad And Kubernetes. The reason is because a healthcheck is a « white box » concept in Docker (e.g. inside the container), while being a « black box » process on others.