Declare build-args in build.yml

This allows multiple build flavors for a single codebase, without
sacrificing reproducible builds. The build-args are set in build.yml,
which is typically under the source control (if it is not, then no
reproducible builds are possible anyways). Meaning that mutating
build-args would result in setting "dirty" flag.

Intended use of this commit is to switch between build flavors by
specifying a different yaml file (presumably also under the version
control)  by  `-build-yml` option.

Because it is impossible to build a final image from packages in
cache, the test for this feature relies on the `RUN echo $build-arg`
output during the `pkg build` process.

Signed-off-by: Yuri Volchkov <yuri@zededa.com>

src/cmd/linuxkit/pkglib/build.go

@@ -257,6 +257,12 @@ func (p Pkg) Build(bos ...BuildOpt) error {
 		args = append(args, "--label=org.mobyproject.linuxkit.version="+version.Version)
 		args = append(args, "--label=org.mobyproject.linuxkit.revision="+version.GitCommit)
 
+		if p.buildArgs != nil {
+			for _, buildArg := range *p.buildArgs {
+				args = append(args, "--build-arg", buildArg)
+			}
+		}
+
 		// build for each arch and save in the linuxkit cache
 		for _, platform := range bo.platforms {
 			desc, err := p.buildArch(d, c, platform.Architecture, args, writer, bo)

src/cmd/linuxkit/pkglib/pkglib.go

@@ -26,6 +26,7 @@ type pkgInfo struct {
 	Network      bool              `yaml:"network"`
 	DisableCache bool              `yaml:"disable-cache"`
 	Config       *moby.ImageConfig `yaml:"config"`
+	BuildArgs    *[]string         `yaml:"buildArgs,omitempty"`
 	Depends      struct {
 		DockerImages struct {
 			TargetDir string   `yaml:"target-dir"`
@@ -54,6 +55,7 @@ type Pkg struct {
 	trust         bool
 	cache         bool
 	config        *moby.ImageConfig
+	buildArgs     *[]string
 	dockerDepends dockerDepends
 
 	// Internal state
@@ -260,6 +262,7 @@ func NewFromCLI(fs *flag.FlagSet, args ...string) ([]Pkg, error) {
 			network:       pi.Network,
 			cache:         !pi.DisableCache,
 			config:        pi.Config,
+			buildArgs:     pi.BuildArgs,
 			dockerDepends: dockerDepends,
 			dirty:         dirty,
 			path:          pkgPath,