test: refactor client tests

should not be explicitly telling what they are testing and the requirements
fixed the test data so that it doesn't contain discrepancies
fully reset the mock client (no shared state) and only support the calls that is expected

pkg/container/mocks/data/containers.json

@@ -2,7 +2,55 @@
   {
     "Id": "ae8964ba86c7cd7522cf84e09781343d88e0e3543281c747d88b27e246578b65",
     "Names": [
-      "/watchtower-test"
+      "/watchtower-stopped"
+    ],
+    "Image": "containrrr/watchtower:latest",
+    "ImageID": "sha256:4dbc5f9c07028a985e14d1393e849ea07f68804c4293050d5a641b138db72daa",
+    "Command": "/watchtower",
+    "Created": 1554925882,
+    "Ports": [],
+    "Labels": {
+      "com.centurylinklabs.watchtower": "true"
+    },
+    "State": "exited",
+    "Status": "Exited (1) 6 days ago",
+    "HostConfig": {
+      "NetworkMode": "default"
+    },
+    "NetworkSettings": {
+      "Networks": {
+        "bridge": {
+          "IPAMConfig": null,
+          "Links": null,
+          "Aliases": null,
+          "NetworkID": "8fcfd56fa9203bafa98510abb08bff66ad05bef5b6e97d158cbae3397e1e065e",
+          "EndpointID": "",
+          "Gateway": "",
+          "IPAddress": "",
+          "IPPrefixLen": 0,
+          "IPv6Gateway": "",
+          "GlobalIPv6Address": "",
+          "GlobalIPv6PrefixLen": 0,
+          "MacAddress": "",
+          "DriverOpts": null
+        }
+      }
+    },
+    "Mounts": [
+      {
+        "Type": "bind",
+        "Source": "/var/run/docker.sock",
+        "Destination": "/var/run/docker.sock",
+        "Mode": "",
+        "RW": true,
+        "Propagation": "rprivate"
+      }
+    ]
+  },
+  {
+    "Id": "3d88e0e3543281c747d88b27e246578b65ae8964ba86c7cd7522cf84e0978134",
+    "Names": [
+      "/watchtower-running"
     ],
     "Image": "containrrr/watchtower:latest",
     "ImageID": "sha256:4dbc5f9c07028a985e14d1393e849ea07f68804c4293050d5a641b138db72daa",
@@ -13,7 +61,7 @@
       "com.centurylinklabs.watchtower": "true"
     },
     "State": "running",
-    "Status": "Exited (1) 6 days ago",
+    "Status": "Up 3 days",
     "HostConfig": {
       "NetworkMode": "default"
     },