Add aliases to Kamal

Aliases are defined in the configuration file under the `aliases` key.

The configuration is a map of alias name to command. When we run the
command the we just do a literal replacement of the alias with the
string.

So if we have:

```yaml
aliases:
  console: app exec -r console -i --reuse "rails console"
```

Then running `kamal console -r workers` will run the command

```sh
$ kamal app exec -r console -i --reuse "rails console" -r workers
```

Because of the order Thor parses the arguments, this allows us to
override the role from the alias command.

There might be cases where we need to munge the command a bit more but
that would involve getting into Thor command parsing internals,
which are complicated and possibly subject to change.

There's a chance that your aliases could conflict with future built-in
commands, but there's not likely to be many of those and if it happens
you'll get a validation error when you upgrade.

Thanks to @dhnaranjo for the idea!

lib/kamal/configuration/alias.rb

@@ -0,0 +1,15 @@
+class Kamal::Configuration::Alias
+  include Kamal::Configuration::Validation
+
+  attr_reader :name, :command
+
+  def initialize(name, config:)
+    @name, @command = name.inquiry, config.raw_config["aliases"][name]
+
+    validate! \
+      command,
+      example: validation_yml["aliases"]["uname"],
+      context: "aliases/#{name}",
+      with: Kamal::Configuration::Validator::Alias
+  end
+end

lib/kamal/configuration/docs/alias.yml

@@ -0,0 +1,26 @@
+# Aliases
+#
+# Aliases are shortcuts for Kamal commands.
+#
+# For example, for a Rails app, you might open a console with:
+#
+# ```shell
+# kamal app exec -i -r console "rails console"
+# ```
+#
+# By defining an alias, like this:
+aliases:
+  console: app exec -r console -i "rails console"
+# You can now open the console with:
+# ```shell
+# kamal console
+# ```
+
+# Configuring aliases
+#
+# Aliases are defined in the root config under the alias key
+#
+# Each alias is named and can only contain lowercase letters, numbers, dashes and underscores.
+
+aliases:
+  uname: app exec -p -q -r web "uname -a"

lib/kamal/configuration/docs/configuration.yml

@@ -166,3 +166,9 @@ healthcheck:
 # Docker logging configuration, see kamal docs logging
 logging:
   ...
+
+# Aliases
+#
+# Alias configuration, see kamal docs alias
+aliases:
+  ...

lib/kamal/configuration/validator.rb

@@ -13,32 +13,34 @@ class Kamal::Configuration::Validator
 
   private
     def validate_against_example!(validation_config, example)
-      validate_type! validation_config, Hash
+      validate_type! validation_config, example.class
 
-      check_unknown_keys! validation_config, example
+      if example.class == Hash
+        check_unknown_keys! validation_config, example
 
-      validation_config.each do |key, value|
-        next if extension?(key)
-        with_context(key) do
-          example_value = example[key]
+        validation_config.each do |key, value|
+          next if extension?(key)
+          with_context(key) do
+            example_value = example[key]
 
-          if example_value == "..."
-            validate_type! value, *(Array if key == :servers), Hash
-          elsif key == "hosts"
-            validate_servers! value
-          elsif example_value.is_a?(Array)
-            validate_array_of! value, example_value.first.class
-          elsif example_value.is_a?(Hash)
-            case key.to_s
-            when "options", "args"
-              validate_type! value, Hash
-            when "labels"
-              validate_hash_of! value, example_value.first[1].class
+            if example_value == "..."
+              validate_type! value, *(Array if key == :servers), Hash
+            elsif key == "hosts"
+              validate_servers! value
+            elsif example_value.is_a?(Array)
+              validate_array_of! value, example_value.first.class
+            elsif example_value.is_a?(Hash)
+              case key.to_s
+              when "options", "args"
+                validate_type! value, Hash
+              when "labels"
+                validate_hash_of! value, example_value.first[1].class
+              else
+                validate_against_example! value, example_value
+              end
             else
-              validate_against_example! value, example_value
+              validate_type! value, example_value.class
             end
-          else
-            validate_type! value, example_value.class
           end
         end
       end

lib/kamal/configuration/validator/alias.rb

@@ -0,0 +1,15 @@
+class Kamal::Configuration::Validator::Alias < Kamal::Configuration::Validator
+  def validate!
+    super
+
+    name = context.delete_prefix("aliases/")
+
+    if name !~ /\A[a-z0-9_-]+\z/
+      error "Invalid alias name: '#{name}'. Must only contain lowercase letters, alphanumeric, hyphens and underscores."
+    end
+
+    if Kamal::Cli::Main.commands.include?(name)
+      error "Alias '#{name}' conflicts with a built-in command."
+    end
+  end
+end