Copy env files to remote hosts

Setting env variables in the docker arguments requires having them on
the deploy host.

Instead we'll add two new commands `kamal env push` and
`kamal env delete` which will manage copying the environment as .env
files to the remote host.

Docker will pick up the file with `--env-file <path-to-file>`. Env files
will be stored under `<kamal run directory>/env`.

Running `kamal env push` will create env files for each role and
accessory, and traefik if required.

`kamal envify` has been updated to also push the env files.

By avoiding using `kamal envify` and creating the local and remote
secrets manually, you can now avoid accessing secrets needed
for the docker runtime environment locally. You will still need build
secrets.

One thing to note - the Docker doesn't parse the environment variables
in the env file, one result of this is that you can't specify multi-line
values - see https://github.com/moby/moby/issues/12997.

We maybe need to look docker config or docker secrets longer term to get
around this.

Hattip to @kevinmcconnell - this was all his idea.

lib/kamal/utils.rb

@@ -16,14 +16,24 @@ module Kamal::Utils
     end
   end
 
-  # Return a list of shell arguments using the same named argument against the passed attributes,
-  # but redacts and expands secrets.
-  def argumentize_env_with_secrets(env)
-    if (secrets = env["secret"]).present?
-      argumentize("-e", secrets.to_h { |key| [ key, ENV.fetch(key) ] }, sensitive: true) + argumentize("-e", env["clear"])
-    else
-      argumentize "-e", env.fetch("clear", env)
-    end
+  def env_file_with_secrets(env)
+    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 || "\n"
   end
 
   # Returns a list of shell-dashed option arguments. If the value is true, it's treated like a value-less option.
@@ -97,4 +107,12 @@ module Kamal::Utils
   def uncommitted_changes
     `git status --porcelain`.strip
   end
+
+  def docker_env_file_line(key, value)
+    if key.include?("\n") || value.to_s.include?("\n")
+      raise ArgumentError, "docker env file format does not support newlines in keys or values, key: #{key}"
+    end
+
+    "#{key.to_s}=#{value.to_s}\n"
+  end
 end