Use current OS as a default

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

Author: gabriel-samfira

client/llb/fileop.go

@@ -5,6 +5,7 @@ import (
 	_ "crypto/sha256" // for opencontainers/go-digest
 	"os"
 	"path/filepath"
+	"runtime"
 	"strconv"
 	"strings"
 	"time"
@@ -771,7 +772,8 @@ func (f *FileOp) Marshal(ctx context.Context, c *Constraints) (digest.Digest, []
 			}
 		}
 
-		var platform string
+		// Assume that we're building an image for the same OS we're running on.
+		platform := runtime.GOOS
 		if f.constraints.Platform != nil {
 			platform = f.constraints.Platform.OS
 		}

client/llb/meta.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"net"
+	"runtime"
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/google/shlex"
@@ -76,7 +77,7 @@ func dirf(value string, replace bool, v ...interface{}) StateOption {
 	}
 	return func(s State) State {
 		return s.withValue(keyDir, func(ctx context.Context, c *Constraints) (interface{}, error) {
-			var platform string
+			platform := runtime.GOOS
 			if c != nil && c.Platform != nil {
 				platform = c.Platform.OS
 			}

frontend/dockerfile/dockerfile2llb/convert.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"path"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -994,7 +995,7 @@ func dispatchRun(d *dispatchState, c *instructions.RunCommand, proxy *llb.ProxyE
 }
 
 func dispatchWorkdir(d *dispatchState, c *instructions.WorkdirCommand, commit bool, opt *dispatchOpt) error {
-	var platformOS string
+	platformOS := runtime.GOOS
 	if d != nil && d.platform != nil {
 		platformOS = d.platform.OS
 	}
@@ -1032,7 +1033,7 @@ func dispatchWorkdir(d *dispatchState, c *instructions.WorkdirCommand, commit bo
 }
 
 func dispatchCopy(d *dispatchState, cfg copyConfig) error {
-	var platformOS string
+	platformOS := runtime.GOOS
 	if d.platform != nil {
 		platformOS = d.platform.OS
 	}

solver/llbsolver/ops/exec.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"os"
 	"path"
+	"runtime"
 	"sort"
 	"strings"
 
@@ -260,7 +261,7 @@ func (e *ExecOp) Exec(ctx context.Context, g session.Group, inputs []solver.Resu
 		}
 	}
 
-	var platformOS string
+	platformOS := runtime.GOOS
 	if e.platform != nil {
 		platformOS = e.platform.OS
 	}