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/cli/server_test.rb

@@ -21,7 +21,7 @@ class CliServerTest < CliTestCase
   test "bootstrap install as root user" do
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:docker, "-v", raise_on_non_zero_exit: false).returns(false).at_least_once
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with('[ "${EUID:-$(id -u)}" -eq 0 ] || command -v sudo >/dev/null || command -v su >/dev/null', raise_on_non_zero_exit: false).returns(true).at_least_once
-    SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:curl, "-fsSL", "https://get.docker.com", "|", :sh).at_least_once
+    SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:sh, "-c", "'curl -fsSL https://get.docker.com || wget -O - https://get.docker.com || echo \"exit 1\"'", "|", :sh).at_least_once
     SSHKit::Backend::Abstract.any_instance.expects(:execute).with(:mkdir, "-p", ".kamal").returns("").at_least_once
 
     run_command("bootstrap").tap do |output|

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