Document that ENV vars are not automatically updated

[thaJeztah]

Unlike the entries in /etc/hosts, environment-variables for linked containers are not automatically updated if the linked container is restarted.

This adds a note to the documentation in; https://docs.docker.com/userguide/dockerlinks/#environment-variables and https://docs.docker.com/reference/run/#env-environment-variables to make users aware that this is the case and recommends them to use the /etc/hosts entries in stead.

I added this change because users were expecting environment variables to be updated automatically as well (#10164).

NOTE;
I decided not to add a note to the brand-new know issues section; let me know if that is desired.

[jamtur01]

LGTM Ping @SvenDowideit @fredlf

[SvenDowideit]

LGTM

[thaJeztah]

Thanks!

[mikehaertl]

Thanks for adding this note, I didn't know that. But now I wonder, if the docs aren't contradictory in some way. Because in the next paragraph it says:

You can use these environment variables to configure your applications to connect to the database on the db container.

I think this recommendation no longer holds true, right? Actually I wonder, what use those env variables are anyway, if you can't really rely on their content. Shouldn't they be removed completely and marked as deprecated in the manual?

[thaJeztah]

The environment variables predate the host entries and, yes, they don't offer very much because (for example) you need to know the port number of the service in order to find its IP-address. It's likely the environment vars will disappear in the future.

Some people prefer env-vars, however (it's a common way to pass configuration settings) and to stay backwards-compatible, they cannot be removed (at least not until Docker 2.0, whenever that is).