Link a build's bases into its repository before pushing it (issue 694)

A runtime on containerd's image store, the default on a fresh docker 29,
need not hold the compressed layers of a base it built FROM: a layer it
had unpacked already is never fetched. Its push then fails on that layer
although the registry holds it under the base's mirror. A push reads
locally only what the target repository lacks, so mount the base's blobs
there first.

internal/builder/builder.go

@@ -1076,6 +1076,16 @@ func one(ctx context.Context, run Runner, publish Publisher,
 			return catalogue.Built{}, fmt.Errorf("%s: building %s failed: %w", module, a.Name, err)
 		}
 		say("image", "built, publishing")
+		// Its bases linked into its repository first, so the push needs none of their blobs from this
+		// machine's store, which may not hold them (novox/hq issue 694). Said and not fatal: a push
+		// from a store that holds them works without, and one that fails says which blob it lacked.
+		if lender, can := publish.(BaseLender); can {
+			if n, err := lender.LendBases(ctx, module+"/"+a.Name, buildArgValues(args)); err != nil {
+				say("image", "its bases could not all be linked into %s/%s before the push: %v", module, a.Name, err)
+			} else if n > 0 {
+				say("image", "%d blob(s) of its bases linked into %s/%s before the push", n, module, a.Name)
+			}
+		}
 		reference, err := publish.PublishImage(ctx, local, module+"/"+a.Name)
 		if err != nil {
 			return catalogue.Built{}, err
@@ -1427,6 +1437,19 @@ func standingOn(ctx context.Context, manifest catalogue.Manifest, held map[strin
 	return args, resolved, nil
 }
 
+// buildArgValues is what each --build-arg of a build was handed: the bases, as this mesh holds them.
+func buildArgValues(args []string) []string {
+	var values []string
+	for i := 0; i+1 < len(args); i += 2 {
+		if args[i] == "--build-arg" {
+			if _, value, has := strings.Cut(args[i+1], "="); has {
+				values = append(values, value)
+			}
+		}
+	}
+	return values
+}
+
 // compile runs a module's own code through its toolchain, and says where the result is.
 //
 // **In the module's own directory, under the path the toolchain expects.** A module is compiled