use SYFT_LOG_FILE env var (#805)

* use SYFT_LOG_FILE

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* enable debug logs when SYFT_LOG_FILE is set

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* set log.file and add tests

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* test log file in temp directory

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* add note on binding refactor

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

* remove unused function

Signed-off-by: Jonas Galvão Xavier <jonas.agx@gmail.com>

test/cli/root_cmd_test.go

@@ -1,10 +1,13 @@
 package cli
 
 import (
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
 	"github.com/sergi/go-diff/diffmatchpatch"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestRootCmdAliasesToPackagesSubcommand(t *testing.T) {
@@ -114,3 +117,47 @@ func TestPersistentFlags(t *testing.T) {
 		})
 	}
 }
+
+func TestLogFile(t *testing.T) {
+	request := "docker-archive:" + getFixtureImage(t, "image-pkg-coverage")
+
+	envLogFile := filepath.Join(os.TempDir(), "a-pretty-log-file.log")
+	t.Logf("log file path: %s", envLogFile)
+
+	tests := []struct {
+		name       string
+		args       []string
+		env        map[string]string
+		assertions []traitAssertion
+		cleanup    func()
+	}{
+		{
+			name: "env-var-log-file-name",
+			args: []string{"-vv", request},
+			env:  map[string]string{"SYFT_LOG_FILE": envLogFile},
+			assertions: []traitAssertion{
+				func(tb testing.TB, stdout, stderr string, rc int) {
+					tb.Helper()
+					_, err := os.Stat(envLogFile)
+					assert.NoError(t, err)
+				},
+			},
+			cleanup: func() { assert.NoError(t, os.Remove(envLogFile)) },
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Cleanup(test.cleanup)
+
+			cmd, stdout, stderr := runSyft(t, test.env, test.args...)
+			for _, traitFn := range test.assertions {
+				traitFn(t, stdout, stderr, cmd.ProcessState.ExitCode())
+			}
+			if t.Failed() {
+				t.Log("STDOUT:\n", stdout)
+				t.Log("STDERR:\n", stderr)
+				t.Log("COMMAND:", strings.Join(cmd.Args, " "))
+			}
+		})
+	}
+}