Always send the clear env to the container

Secret and clear env variables have different lifecycles. The clear ones
are part of the repo, so it makes sense to always deploy them with the
rest of the repo.

The secret ones are external so we can't be sure that they are up to
date, therefore they require an explicit push via `envify` or `env push`.

We'll keep the env file, but now it just contains secrets. The clear
values are passed directly to `docker run`.

lib/kamal/configuration/env.rb

@@ -0,0 +1,40 @@
+class Kamal::Configuration::Env
+  attr_reader :secrets_keys, :clear, :secrets_file
+  delegate :argumentize, to: Kamal::Utils
+
+  def self.from_config(config:, secrets_file: nil)
+    secrets_keys = config.fetch("secret", [])
+    clear = config.fetch("clear", config.key?("secret") ? {} : config)
+
+    new clear: clear, secrets_keys: secrets_keys, secrets_file: secrets_file
+  end
+
+  def initialize(clear:, secrets_keys:, secrets_file:)
+    @clear = clear
+    @secrets_keys = secrets_keys
+    @secrets_file = secrets_file
+  end
+
+  def args
+    [ "--env-file", secrets_file, *argumentize("--env", clear) ]
+  end
+
+  def secrets_io
+    StringIO.new(Kamal::EnvFile.new(secrets).to_s)
+  end
+
+  def secrets
+    @secrets ||= secrets_keys.to_h { |key| [ key, ENV.fetch(key) ] }
+  end
+
+  def secrets_directory
+    File.dirname(secrets_file)
+  end
+
+  def merge(other)
+    self.class.new \
+      clear: @clear.merge(other.clear),
+      secrets_keys: @secrets_keys | other.secrets_keys,
+      secrets_file: secrets_file
+  end
+end