[NON-FUNCTIONAL] Get golang version from go module

Dockerfile

# see https://github.com/jtagcat/dotfiles/blob/main/scripts/template/gobuild.Dockerfile
# stuck on https://github.com/moby/moby/issues/29110

FROM alpine
COPY getGoModVersion.sh .
COPY go.mod .
ENVCMD gover ./dockerGoMod.sh | sed -n '1 p'

FROM golang:$ver AS builder

# ...

ENVCMD basename dockerGoMod.sh | sed -n '3 p'
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o $basename

ENVCMD module dockerGoMod.sh | sed -n '2 p'
LABEL org.opencontainers.image.source="https://$module"

# ...

COPY --from=builder /wd/$basename ./
CMD ["./$basename"]

dockerGoMod.sh

#!/usr/bin/env bash
set -eo pipefail

file="go.mod"
if [[ -n "$1" ]]; then
	file="$1"
fi

if [[ ! -r "$file" ]]; then
	echo "error: file '${file}' not readable"
	exit 1
fi

matches=0
while read -r line; do
	if [[ "$line" =~ ^go[[:space:]].* ]]; then
		# begins with "go "
		v="$(xargs <<< "${line:3}")"

		if [[ "$matches" != 0 ]]; then
			>&2 echo "error: multiple versions found, 2nd is '${v}'"
			exit 1
		fi

		echo "${v}"
		((matches++))
	fi
done <go.mod

if [[ "$matches" == 0 ]]; then
	>&2 echo "error: no version found"
	exit 1
fi


matches=0
while read -r line; do
	if [[ "$line" =~ ^module[[:space:]].* ]]; then
		# begins with "module "
		v="$(xargs <<< "${line:7}")"

		if [[ "$matches" != 0 ]]; then
			>&2 echo "error: multiple module names found, 2nd is '${v}'"
			exit 1
		fi

		echo "${v}"
        basename "${v}"
		((matches++))
	fi
done <go.mod

if [[ "$matches" == 0 ]]; then
	>&2 echo "error: no module name found"
	exit 1
fi