build: move SOURCE_DATE_EPOCH parsing into option generation

This allows the build package code to become more generic, and also
ensures that when the environment variables are not propogated (in the
case of the remote controller), that we can still correctly set
SOURCE_DATE_EPOCH.

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/build.go

@@ -604,13 +604,6 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 		}
 	}
 
-	// Propagate SOURCE_DATE_EPOCH from the client env
-	if v := os.Getenv("SOURCE_DATE_EPOCH"); v != "" {
-		if _, ok := so.FrontendAttrs["build-arg:SOURCE_DATE_EPOCH"]; !ok {
-			so.FrontendAttrs["build-arg:SOURCE_DATE_EPOCH"] = v
-		}
-	}
-
 	// set platforms
 	if len(opt.Platforms) != 0 {
 		pp := make([]string, len(opt.Platforms))

commands/build.go

@@ -100,6 +100,13 @@ func (o *buildOptions) toControllerOptions() (controllerapi.BuildOptions, error)
 		Opts:           &o.CommonOptions,
 	}
 
+	// TODO: extract env var parsing to a method easily usable by library consumers
+	if v := os.Getenv("SOURCE_DATE_EPOCH"); v != "" {
+		if _, ok := opts.BuildArgs["SOURCE_DATE_EPOCH"]; !ok {
+			opts.BuildArgs["SOURCE_DATE_EPOCH"] = v
+		}
+	}
+
 	inAttests := append([]string{}, o.attests...)
 	if o.provenance != "" {
 		inAttests = append(inAttests, buildflags.CanonicalizeAttest("provenance", o.provenance))