Install docker with curl or wget

If curl is not available to download the docker install script, try
with wget instead.

If neither is available or both fail, return a simple failing script
so that we don't carry on regardless.

Fixes: https://github.com/basecamp/kamal/issues/395

lib/kamal/commands/base.rb

@@ -70,6 +70,10 @@ module Kamal::Commands
         [ :xargs, command ].flatten
       end
 
+      def shell(command)
+        [ :sh, "-c", "'#{command.flatten.join(" ").gsub("'", "'\\''")}'" ]
+      end
+
       def docker(*args)
         args.compact.unshift :docker
       end

lib/kamal/commands/docker.rb

@@ -1,7 +1,7 @@
 class Kamal::Commands::Docker < Kamal::Commands::Base
   # Install Docker using the https://github.com/docker/docker-install convenience script.
   def install
-    pipe [ :curl, "-fsSL", "https://get.docker.com" ], :sh
+    pipe get_docker, :sh
   end
 
   # Checks the Docker client version. Fails if Docker is not installed.
@@ -18,4 +18,13 @@ class Kamal::Commands::Docker < Kamal::Commands::Base
   def superuser?
     [ '[ "${EUID:-$(id -u)}" -eq 0 ] || command -v sudo >/dev/null || command -v su >/dev/null' ]
   end
+
+  private
+    def get_docker
+      shell \
+        any \
+          [ :curl, "-fsSL", "https://get.docker.com" ],
+          [ :wget, "-O -", "https://get.docker.com" ],
+          [ :echo, "\"exit 1\"" ]
+    end
 end