Filter out docker build-arg option if no BASE_IMAGE (#3259) (#3261)

* Filter out docker build-arg option if no BASE_IMAGE (#3259)

If the 'BASE_IMAGE' var is not defined in the environment, filter out
the build template entry for Goreleaser to invoke `docker build[x]` so
that we avoid calling `docker build` with an invliad `--build-arg= ` (no
value) option, which was causing the build to fail.

Signed-off-by: Rich Scott <[email protected]>

* Use golang.org/x/exp/slices.Delete

We are using Go 1.20 for builds, so use golang.org/x/exp/slices.Delete
instead of slices.DeleteFunc (new in Go 1.21 stdlib).

Signed-off-by: Rich Scott <[email protected]>

---------

Signed-off-by: Rich Scott <[email protected]>

magefiles/goreleaser.go

@@ -3,8 +3,11 @@ package main
 import (
 	"fmt"
 	"os"
+	"regexp"
 	"time"
 
+	"golang.org/x/exp/slices"
+
 	goreleaserConfig "github.com/goreleaser/goreleaser/pkg/config"
 	"github.com/magefile/mage/sh"
 	"github.com/pkg/errors"
@@ -51,8 +54,30 @@ func goreleaserWriteMinimalReleaseConfig(dockerIds ...string) error {
 	dockerIdsToBuild := set(dockerIds)
 	dockersById := make(map[string]goreleaserConfig.Docker)
 	buildIds := make(map[string]bool)
+
+	// BASE_IMAGE_ARG is derived from BASE_IMAGE - see .goreleaser.yml
+	buildArgBaseRE := regexp.MustCompile("--build-arg=.*BASE_IMAGE_ARG")
+
 	for _, docker := range config.Dockers {
 		if dockerIdsToBuild[docker.ID] {
+			// Goreleaser constructs a faulty `docker build` command if the environment
+			// variable BASE_IMAGE is not defined - it constructs a `--build-arg= ` option
+			// (no value given) and `docker build` immediately errors. As a work-around
+			// we filter out that whole template option entry if BASE_IMAGE is not defined;
+			// see .goreleaser.yml for how it's used. Goreleaser does not support conditional
+			// templating of YAML in its 'dockers' section, so we filter it out here.
+			if os.Getenv("BASE_IMAGE") == "" {
+				deleteIdx := -1
+				for idx, tpl := range docker.BuildFlagTemplates {
+					if buildArgBaseRE.MatchString(tpl) {
+						deleteIdx = idx
+						break
+					}
+				}
+
+				docker.BuildFlagTemplates = slices.Delete(docker.BuildFlagTemplates, deleteIdx, deleteIdx+1)
+			}
+
 			dockersById[docker.ID] = docker
 			for _, id := range docker.IDs {
 				buildIds[id] = true