This patch fixes a bug where labels use the same behavior as --env, resulting
in a value to be copied from environment variables with the same name as the
label if no value is set (i.e. a simple key, no = sign, no value).

An earlier pull request addressed similar cases for docker run;
2b17f4c, but this did not address the
same situation for (e.g.) docker service create.

Digging in history for this bug, I found that use of the ValidateEnv
function for labels was added in the original implementation of the labels feature in
moby/moby@abb5e9a#diff-ae476143d40e21ac0918630f7365ed3cR34

However, the design never intended it to expand environment variables,
and use of this function was either due to either a "copy/paste" of the
equivalent --env flags, or a misunderstanding (the name ValidateEnv does
not communicate that it also expands environment variables), and the existing
ValidateLabel was designed for engine labels (which required a value to
be set).

Following the initial implementation, other parts of the code followed
the same (incorrect) approach, therefore leading the bug to be introduced
in services as well.

This patch:

• updates the ValidateLabel to match the expected validation
  rules (this function is no longer used since 31dc5c0),
  and the daemon has its own implementation)
• corrects various locations in the code where ValidateEnv was used instead of ValidateLabel.

Before this patch:

export SOME_ENV_VAR=I_AM_SOME_ENV_VAR
docker service create --label SOME_ENV_VAR --tty --name test busybox

docker service inspect --format '{{json .Spec.Labels}}' test
{"SOME_ENV_VAR":"I_AM_SOME_ENV_VAR"}

After this patch:

export SOME_ENV_VAR=I_AM_SOME_ENV_VAR
docker service create --label SOME_ENV_VAR --tty --name test busybox

docker container inspect --format '{{json .Config.Labels}}' test
{"SOME_ENV_VAR":""}

Add back validation for invalid label values on containers

This adds validation to docker container run / docker container create;

Validation of labels provided through flags was removed in 31dc5c0,
after the validation was changed to fix labels without values, and to prevent
labels from being expanded with environment variables in 2b17f4c

However, now empty label names from files (--label-file) followed different
validation rules than labels passed through --label.

This patch adds back minimal validation for labels passed through the command-line

Before this patch:

docker container create \
--name label \
--label==with-leading-equal-sign \
--label=without-value \
--label=somelabel=somevalue \
--label " = " \
--label=with-quotes-in-value='{"foo"}' \
--label='with"quotes"in-key=test' \
busybox

docker container inspect --format '{{json .Config.Labels}}' label

{
"": "with-leading-equal-sign",
" ": " ",
"somelabel": "somevalue",
"with\"quotes\"in-key": "test",
"with-quotes-in-value": "{\"foo\"}",
"without-value": ""
}

After this patch:

docker container create \
--name label \
--label==with-leading-equal-sign \
--label=without-value \
--label=somelabel=somevalue \
--label " = " \
--label=with-quotes-in-value='{"foo"}' \
--label='with"quotes"in-key=test' \
busybox

invalid argument "=with-leading-equal-sign" for "-l, --label" flag: invalid label format: "=with-leading-equal-sign"