Update BUILD_DATE format to include seconds

* Comply with all standards (RFC 3339, ISO 8601, and HTML).
* Reduce ambiguity.
* Ensure a fully compliant `org.opencontainers.image.created`. (fix artifacthub error)

build.sh

@@ -223,7 +223,7 @@ fi
 ###
 # Variables for labelling the docker image
 ###
-BUILD_DATE="$(date -u '+%Y-%m-%dT%H:%M+00:00')"
+BUILD_DATE="$(date -u '+%Y-%m-%dT%H:%M:%S+00:00')"
 
 if [ -d ".git" ] && [ -z "${SKIP_GIT}" ]; then
   GIT_REF="$(git rev-parse HEAD)"