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

test/commands/docker_test.rb

@@ -9,7 +9,7 @@ class CommandsDockerTest < ActiveSupport::TestCase
   end
 
   test "install" do
-    assert_equal "curl -fsSL https://get.docker.com | sh", @docker.install.join(" ")
+    assert_equal "sh -c 'curl -fsSL https://get.docker.com || wget -O - https://get.docker.com || echo \"exit 1\"' | sh", @docker.install.join(" ")
   end
 
   test "installed?" do