docker-cli: add docker contexts support

Adds docker-cli.contexts support. This allows declarative configuration
of [docker
contexts](https://docs.docker.com/engine/manage-resources/contexts/).

tests/modules/programs/docker-cli/example-contexts.nix

@@ -0,0 +1,36 @@
+{
+  config,
+  ...
+}:
+{
+  programs.docker-cli = {
+    enable = true;
+
+    configDir = ".docker2";
+
+    contexts = {
+      example = {
+        Metadata = {
+          Description = "example1";
+        };
+        Endpoints = {
+          docker = {
+            Host = "unix://example2";
+          };
+        };
+      };
+    };
+  };
+
+  nmt.script =
+    let
+      cfgDocker = config.programs.docker-cli;
+      configTestPath = "home-files/${cfgDocker.configDir}/contexts/meta/50d858e0985ecc7f60418aaf0cc5ab587f42c2570a884095a9e8ccacd0f6545c/meta.json";
+    in
+    ''
+      assertPathNotExists home-files/.docker/config.json
+      assertFileExists ${configTestPath}
+      assertFileContent ${configTestPath} \
+        ${./example-contexts.json}
+    '';
+}