Can't build docker image (ubuntu)

[leonidcan]

Can't build docker image (ubuntu)

/docker-ipv6nat# docker build .
Sending build context to Docker daemon 61.44kB
Step 1/17 : FROM --platform=$BUILDPLATFORM golang:1.16.6-alpine3.14 AS build
failed to parse platform : "" is an invalid component of "": platform specifier component must match "^[A-Za-z0-9_-]+$": invalid argument

[SuperSandro2000]

See https://github.com/robbertkl/docker-ipv6nat/blob/master/push.sh#L22-L29 how it is build.