diff --git a/opts/file.go b/opts/file.go
index d1089ea41d..535839aa7a 100644
--- a/opts/file.go
+++ b/opts/file.go
@@ -12,7 +12,7 @@ import (
 
 const whiteSpaces = " \t"
 
-func parseKeyValueFile(filename string, emptyFn func(string) (string, bool)) ([]string, error) {
+func parseKeyValueFile(filename string, lookupFn func(string) (string, bool)) ([]string, error) {
 	fh, err := os.Open(filename)
 	if err != nil {
 		return []string{}, err
@@ -21,45 +21,48 @@ func parseKeyValueFile(filename string, emptyFn func(string) (string, bool)) ([]
 
 	lines := []string{}
 	scanner := bufio.NewScanner(fh)
-	currentLine := 0
 	utf8bom := []byte{0xEF, 0xBB, 0xBF}
-	for scanner.Scan() {
+	for currentLine := 1; scanner.Scan(); currentLine++ {
 		scannedBytes := scanner.Bytes()
 		if !utf8.Valid(scannedBytes) {
-			return []string{}, fmt.Errorf("env file %s contains invalid utf8 bytes at line %d: %v", filename, currentLine+1, scannedBytes)
+			return []string{}, fmt.Errorf("env file %s contains invalid utf8 bytes at line %d: %v", filename, currentLine, scannedBytes)
 		}
 		// We trim UTF8 BOM
-		if currentLine == 0 {
+		if currentLine == 1 {
 			scannedBytes = bytes.TrimPrefix(scannedBytes, utf8bom)
 		}
-		// trim the line from all leading whitespace first
+		// trim the line from all leading whitespace first. trailing whitespace
+		// is part of the value, and is kept unmodified.
 		line := strings.TrimLeftFunc(string(scannedBytes), unicode.IsSpace)
-		currentLine++
-		// line is not empty, and not starting with '#'
-		if len(line) > 0 && !strings.HasPrefix(line, "#") {
-			variable, value, hasValue := strings.Cut(line, "=")
 
-			// trim the front of a variable, but nothing else
-			variable = strings.TrimLeft(variable, whiteSpaces)
-			if strings.ContainsAny(variable, whiteSpaces) {
-				return []string{}, fmt.Errorf("variable '%s' contains whitespaces", variable)
-			}
-			if len(variable) == 0 {
-				return []string{}, fmt.Errorf("no variable name on line '%s'", line)
-			}
+		if len(line) == 0 || line[0] == '#' {
+			// skip empty lines and comments (lines starting with '#')
+			continue
+		}
 
-			if hasValue {
-				// pass the value through, no trimming
-				lines = append(lines, variable+"="+value)
-			} else {
-				var present bool
-				if emptyFn != nil {
-					value, present = emptyFn(line)
-				}
-				if present {
-					// if only a pass-through variable is given, clean it up.
-					lines = append(lines, strings.TrimSpace(variable)+"="+value)
-				}
+		key, _, hasValue := strings.Cut(line, "=")
+		if len(key) == 0 {
+			return []string{}, fmt.Errorf("no variable name on line '%s'", line)
+		}
+
+		// leading whitespace was already removed from the line, but
+		// variables are not allowed to contain whitespace or have
+		// trailing whitespace.
+		if strings.ContainsAny(key, whiteSpaces) {
+			return []string{}, fmt.Errorf("variable '%s' contains whitespaces", key)
+		}
+
+		if hasValue {
+			// key/value pair is valid and has a value; add the line as-is.
+			lines = append(lines, line)
+			continue
+		}
+
+		if lookupFn != nil {
+			// No value given; try to look up the value. The value may be
+			// empty but if no value is found, the key is omitted.
+			if value, found := lookupFn(line); found {
+				lines = append(lines, key+"="+value)
 			}
 		}
 	}