Implement 'healthcheck' option

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

tests/fixtures/healthcheck/docker-compose.yml

@@ -0,0 +1,24 @@
+version: "3"
+services:
+  passes:
+    image: busybox
+    command: top
+    healthcheck:
+      test: "/bin/true"
+      interval: 1s
+      timeout: 30m
+      retries: 1
+
+  fails:
+    image: busybox
+    command: top
+    healthcheck:
+      test: ["CMD", "/bin/false"]
+      interval: 2.5s
+      retries: 2
+
+  disabled:
+    image: busybox
+    command: top
+    healthcheck:
+      disable: true

tests/fixtures/v3-full/docker-compose.yml

@@ -29,7 +29,7 @@ services:
         constraints: [node=foo]
 
     healthcheck:
-      command: cat /etc/passwd
+      test: cat /etc/passwd
       interval: 10s
       timeout: 1s
       retries: 5