support multiline labels and empty labels

src/main/java/io/fabric8/maven/docker/assembly/DockerFileBuilder.java

@@ -60,10 +60,10 @@ public class DockerFileBuilder {
     private List<String> runCmds = new ArrayList<>();
 
     // environment
-    private Map<String,String> envEntries = new HashMap<>();
+    private Map<String,String> envEntries = new LinkedHashMap<>();
 
     // image labels
-    private Map<String, String> labels = new HashMap<>();
+    private Map<String, String> labels = new LinkedHashMap<>();
 
     // exposed volumes
     private List<String> volumes = new ArrayList<>();
@@ -225,14 +225,48 @@ private void addMap(StringBuilder b,DockerFileKeyword keyword, Map<String,String
             String entries[] = new String[map.size()];
             int i = 0;
             for (Map.Entry<String, String> entry : map.entrySet()) {
-                entries[i++] = quote(entry.getKey()) + "=" + quote(entry.getValue());
+                entries[i++] = createKeyValue(entry.getKey(), entry.getValue());
             }
             keyword.addTo(b, entries);
         }
     }
 
-    private String quote(String value) {
-        return StringUtils.quoteAndEscape(value,'"');
+    /**
+     * Escape any slashes, quotes, and newlines int the value.  If any escaping occurred, quote the value.
+     * @param key The key
+     * @param value The value
+     * @return Escaped and quoted key="value"
+     */
+    private String createKeyValue(String key, String value) {
+        StringBuilder sb = new StringBuilder();
+        // no quoting the key; "Keys are alphanumeric strings which may contain periods (.) and hyphens (-)"
+        sb.append(key).append('=');
+        if (value == null || value.isEmpty()) {
+            return sb.append("\"\"").toString();
+        }
+	StringBuffer valBuf = new StringBuffer();
+	boolean escaped = false;
+        for (int i = 0; i < value.length(); ++i) {
+            char c = value.charAt(i);
+            switch (c) {
+                case '"':
+                case '\n':
+                case '\\':
+                    escaped = true;
+                    // escape the character
+                    valBuf.append('\\');
+                    // fall into writing the character
+                default:
+                    valBuf.append(c);
+            }
+        }
+        if (escaped) {
+            // need to keep quotes
+            sb.append('"').append(valBuf.toString()).append('"');
+        } else {
+            sb.append(value);
+        }
+        return sb.toString();
     }
 
     private void addPorts(StringBuilder b) {
@@ -396,7 +430,6 @@ public DockerFileBuilder env(Map<String, String> values) {
     public DockerFileBuilder labels(Map<String,String> values) {
         if (values != null) {
             this.labels.putAll(values);
-            validateMap(labels);
         }
         return this;
     }

src/test/java/io/fabric8/maven/docker/assembly/DockerFileBuilderTest.java

@@ -36,7 +36,22 @@ public void testBuildDockerFile() throws Exception {
         String expected = loadFile("docker/Dockerfile.test");
         assertEquals(expected, stripCR(dockerfileContent));
     }
-
+
+    @Test
+    public void testBuildDockerFileMultilineLabel() throws Exception {
+        Arguments a = Arguments.Builder.get().withParam("c1").withParam("c2").build();
+        String dockerfileContent = new DockerFileBuilder()
+                .add("/src", "/dest")
+                .baseImage("image")
+                .cmd(a)
+                .labels(ImmutableMap.of("key", "unquoted",
+                        "flag", "",
+                        "some-json", "{\n  \"key\": \"value\"\n}\n"))
+                .content();
+        String expected = loadFile("docker/Dockerfile.multiline_label.test");
+        assertEquals(expected, stripCR(dockerfileContent));
+    }
+
     @Test
     public void testBuildDockerFileUDPPort() throws Exception {
         Arguments a = Arguments.Builder.get().withParam("c1").withParam("c2").build();

src/test/resources/docker/Dockerfile.multiline_label.test

@@ -0,0 +1,7 @@
+FROM image
+LABEL key=unquoted flag="" some-json="{\
+  \"key\": \"value\"\
+}\
+"
+COPY /src /maven/dest
+CMD ["c1","c2"]