Update ARG, ENV and LABEL reference definitions

[thompson-shaun]

Follow-up to suggestion from @thaJeztah in #5427

Update ARG and ENV syntax definitions to clarify that first KV-pair is required but additional ones are optional

[dvdksn]

LGTM but we should also update it for LABEL

[thompson-shaun]

LGTM but we should also update it for LABEL

Added LABEL, great catch @dvdksn 🫡

[thaJeztah]

Ah, thanks! I had the tab still open as reminder, but didn't get round to opening a PR.

Changes LGTM; one more thing (but doesn't have to be in this PR) I was considering, but I still had to read up on the current docs, is to make sure we properly outline the distinction between "value is set" and "no value" (ARG FOO= vs ARG FOO).

Something along the lines of;

A default value can be set, but may be empty. The following example declares the FOO arg with an empty default;

ARG FOO=

If the value is omitted, the default is looked up from environment (ENV) defined in the Dockerfile or its parent images, but can be overridden through the --build-arg command-line option. If no value is found, the ARG is unset.

docker build --progress=plain -<<'EOF'
# syntax=docker/dockerfile:1

# nginx:alpine defines a NGINX_VERSION env-var
FROM nginx:alpine
ARG FOO_UNSET
ARG FOO_EMPTY=
ARG NGINX_VERSION
RUN printenv | grep -E '(FOO|NGINX)'
EOF

#5 [2/2] RUN printenv | grep -E '(FOO|NGINX)'
#5 0.238 FOO_EMPTY=
#5 0.238 NGINX_VERSION=1.27.2

docker build --build-arg NGINX_VERSION=override --progress=plain -<<'EOF'
# syntax=docker/dockerfile:1

# nginx:alpine defines a NGINX_VERSION env-var
FROM nginx:alpine
ARG FOO_UNSET
ARG FOO_EMPTY=
ARG NGINX_VERSION
RUN printenv | grep -E '(FOO|NGINX)'
EOF

#5 [2/2] RUN printenv | grep -E '(FOO|NGINX)'
#5 0.221 FOO_EMPTY=
#5 0.221 NGINX_VERSION=override

[thaJeztah]

LGTM