Publish diagnostic that validates a build stage's name

[rcjsuen]

A build stage may not start with a number or contain symbols. It does allow a _, a - or a . though. It cannot start with one of those symbols either.

FROM alpine AS a_lpi-n.e99

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/1 : FROM alpine AS a_lpi-n.e99
 ---> 7328f6f8b418
Successfully built 7328f6f8b418

FROM scratch AS 1s

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/1 : FROM scratch AS 1s
invalid name for build stage: "1s", name can't start with a number or contain symbols

FROM alpine AS _s

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/1 : FROM alpine AS _s
invalid name for build stage: "_s", name can't start with a number or contain symbols

FROM alpine AS a_lpi-n.e,99

$ docker build .
Sending build context to Docker daemon  86.15MB
Step 1/1 : 
invalid name for build stage: "a_lpi-n.e,99", name can't start with a number or contain symbols