Extract Kamal::EnvFile

Cleaning up the Utils junk drawer.

lib/kamal/env_file.rb

@@ -0,0 +1,41 @@
+# Encode an env hash as a string where secret values have been looked up and all values escaped for Docker.
+class Kamal::EnvFile
+  def initialize(env)
+    @env = env
+  end
+  
+  def to_s
+    env_file = StringIO.new.tap do |contents|
+      if (secrets = @env["secret"]).present?
+        @env.fetch("secret", @env)&.each do |key|
+          contents << docker_env_file_line(key, ENV.fetch(key))
+        end
+
+        @env["clear"]&.each do |key, value|
+          contents << docker_env_file_line(key, value)
+        end
+      else
+        @env.fetch("clear", @env)&.each do |key, value|
+          contents << docker_env_file_line(key, value)
+        end
+      end
+    end.string
+
+    # Ensure the file has some contents to avoid the SSHKIT empty file warning
+    env_file.presence || "\n"
+  end
+
+  alias to_str to_s
+  
+  private
+    def docker_env_file_line(key, value)
+      "#{key.to_s}=#{escape_docker_env_file_value(value)}\n"
+    end
+
+    # Escape a value to make it safe to dump in a docker file.
+    def escape_docker_env_file_value(value)
+      # Doublequotes are treated literally in docker env files
+      # so remove leading and trailing ones and unescape any others
+      value.to_s.dump[1..-2].gsub(/\\"/, "\"")
+    end
+end