Add a kamal pre-build hook with some safety checks

taken directly from bc3

ref: https://app.fizzy.do/5986089/cards/3154

.kamal/hooks/pre-build

@@ -0,0 +1,37 @@
+#!/usr/bin/env ruby
+
+def exit_with_error(message)
+  $stderr.puts message
+  exit 1
+end
+
+def check_branch
+  if ENV["KAMAL_DESTINATION"] == "production"
+    current_branch = `git branch --show-current`.strip
+
+    if current_branch != "main"
+      exit_with_error "Only the `main` branch should be deployed to production, current branch is #{current_branch}. If this is expected, try again with `SKIP_GIT_CHECKS=1` prepended to the command"
+    end
+  end
+end
+
+def check_for_uncommitted_changes
+  if `git status --porcelain`.strip.length != 0
+    exit_with_error "You have uncommitted changes, aborting"
+  end
+end
+
+def check_local_and_remote_heads_match
+  remote_head = `git ls-remote origin --tags $(git branch --show-current) | cut -f1 | head -1`.strip
+  local_head = `git rev-parse HEAD`.strip
+
+  if local_head != remote_head
+    exit_with_error "Remote HEAD #{remote_head}, differs from local HEAD #{local_head}, aborting"
+  end
+end
+
+unless ENV["SKIP_GIT_CHECKS"]
+  check_branch
+  check_for_uncommitted_changes
+  check_local_and_remote_heads_match
+end