Adapting REGEXP to docker requirements

According to the `docker` CLI, when running a container with a wrong name, the following output is shown:
```
> docker run -it --name '$$' alpine
docker: Error response from daemon: Invalid container name ($$), only [a-zA-Z0-9][a-zA-Z0-9_.-] are allowed.
```
Concatenating `container:` to the expected regular expression `[a-zA-Z0-9][a-zA-Z0-9_.-]` helps us rejecting wrong values.

docs/content/en/schemas/v2beta10.json

@@ -992,12 +992,11 @@
         },
         "network": {
           "type": "string",
-          "description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are `host`: use the host's networking stack. `bridge`: use the bridged network configuration. `container:<name|id>`: reuse another container's network stack. `none`: no networking in the container.",
-          "x-intellij-html-description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are <code>host</code>: use the host's networking stack. <code>bridge</code>: use the bridged network configuration. <code>container:&lt;name|id&gt;</code>: reuse another container's network stack. <code>none</code>: no networking in the container.",
+          "description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are `host`: use the host's networking stack. `bridge`: use the bridged network configuration. `none`: no networking in the container.",
+          "x-intellij-html-description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are <code>host</code>: use the host's networking stack. <code>bridge</code>: use the bridged network configuration. <code>none</code>: no networking in the container.",
           "enum": [
             "host",
             "bridge",
-            "container:<name|id>",
             "none"
           ]
         },

docs/content/en/schemas/v2beta11.json

@@ -992,11 +992,12 @@
         },
         "network": {
           "type": "string",
-          "description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are `host`: use the host's networking stack. `bridge`: use the bridged network configuration. `none`: no networking in the container.",
-          "x-intellij-html-description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are <code>host</code>: use the host's networking stack. <code>bridge</code>: use the bridged network configuration. <code>none</code>: no networking in the container.",
+          "description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are `host`: use the host's networking stack. `bridge`: use the bridged network configuration. `container:<name|id>`: reuse another container's network stack. `none`: no networking in the container.",
+          "x-intellij-html-description": "passed through to docker and overrides the network configuration of docker builder. If unset, use whatever is configured in the underlying docker daemon. Valid modes are <code>host</code>: use the host's networking stack. <code>bridge</code>: use the bridged network configuration. <code>container:&lt;name|id&gt;</code>: reuse another container's network stack. <code>none</code>: no networking in the container.",
           "enum": [
             "host",
             "bridge",
+            "container:<name|id>",
             "none"
           ]
         },

pkg/skaffold/schema/validation/validation.go

@@ -189,7 +189,7 @@ func validateDockerNetworkMode(artifacts []*latest.Artifact) (errs []error) {
 		if mode == "none" || mode == "bridge" || mode == "host" {
 			continue
 		}
-		containerRegExp := regexp.MustCompile("container:.+")
+		containerRegExp := regexp.MustCompile("^container:[a-zA-Z0-9][a-zA-Z0-9_.-]*$")
 		if containerRegExp.MatchString(mode) {
 			continue
 		}

pkg/skaffold/schema/validation/validation_test.go

@@ -309,13 +309,54 @@ func TestValidateNetworkMode(t *testing.T) {
 			shouldErr: true,
 		},
 		{
-			description: "container's network stack",
+			description: "wrong container's network stack '-not-valid'",
 			artifacts: []*latest.Artifact{
 				{
 					ImageName: "image/container",
 					ArtifactType: latest.ArtifactType{
 						DockerArtifact: &latest.DockerArtifact{
-							NetworkMode: "Container:unique-identifier",
+							NetworkMode: "Container:-not-valid",
+						},
+					},
+				},
+			},
+			shouldErr: true,
+		},
+		{
+			description: "wrong container's network stack 'fussball'",
+			artifacts: []*latest.Artifact{
+				{
+					ImageName: "image/container",
+					ArtifactType: latest.ArtifactType{
+						DockerArtifact: &latest.DockerArtifact{
+							NetworkMode: "Container:fußball",
+						},
+					},
+				},
+			},
+			shouldErr: true,
+		},
+		{
+			description: "container's network stack 'unique'",
+			artifacts: []*latest.Artifact{
+				{
+					ImageName: "image/container",
+					ArtifactType: latest.ArtifactType{
+						DockerArtifact: &latest.DockerArtifact{
+							NetworkMode: "Container:unique",
+						},
+					},
+				},
+			},
+		},
+		{
+			description: "container's network stack 'unique-id.123'",
+			artifacts: []*latest.Artifact{
+				{
+					ImageName: "image/container",
+					ArtifactType: latest.ArtifactType{
+						DockerArtifact: &latest.DockerArtifact{
+							NetworkMode: "Container:unique-id.123",
 						},
 					},
 				},