The build dev command warns about untracked and uncommitted files

so there's a complete picture of what is being packaged that's not in git.

lib/kamal/cli/build.rb

@@ -116,9 +116,22 @@ def dev
 
     ensure_docker_installed
 
-    uncommitted_changes = Kamal::Git.uncommitted_changes
-    if uncommitted_changes.present?
-      say "WARNING: building with uncommitted changes:\n #{uncommitted_changes}", :yellow
+    docker_included_files = Set.new(Kamal::Docker.included_files)
+    git_uncommitted_files = Set.new(Kamal::Git.uncommitted_files)
+    git_untracked_files = Set.new(Kamal::Git.untracked_files)
+
+    docker_uncommitted_files = docker_included_files & git_uncommitted_files
+    if docker_uncommitted_files.any?
+      say "WARNING: Files with uncommitted changes will be present in the dev container:", :yellow
+      docker_uncommitted_files.each { |f| say "  #{f}", :yellow }
+      say
+    end
+
+    docker_untracked_files = docker_included_files & git_untracked_files
+    if docker_untracked_files.any?
+      say "WARNING: Untracked files will be present in the dev container:", :yellow
+      docker_untracked_files.each { |f| say "  #{f}", :yellow }
+      say
     end
 
     with_env(KAMAL.config.builder.secrets) do

lib/kamal/docker.rb

@@ -0,0 +1,30 @@
+require "tempfile"
+require "open3"
+
+module Kamal::Docker
+  extend self
+  BUILD_CHECK_TAG = "kamal-local-build-check"
+
+  def included_files
+    Tempfile.create do |dockerfile|
+      dockerfile.write(<<~DOCKERFILE)
+        FROM busybox
+        COPY . app
+        WORKDIR app
+        CMD find . -type f | sed "s|^\./||"
+      DOCKERFILE
+      dockerfile.close
+
+      cmd = "docker buildx build -t=#{BUILD_CHECK_TAG} -f=#{dockerfile.path} ."
+      system(cmd) || raise("failed to build check image")
+    end
+
+    cmd = "docker run --rm #{BUILD_CHECK_TAG}"
+    out, err, status = Open3.capture3(cmd)
+    unless status
+      raise "failed to run check image:\n#{err}"
+    end
+
+    out.lines.map(&:strip)
+  end
+end

lib/kamal/git.rb

@@ -24,4 +24,14 @@ def uncommitted_changes
   def root
     `git rev-parse --show-toplevel`.strip
   end
+
+  # returns an array of relative path names of files with uncommitted changes
+  def uncommitted_files
+    `git ls-files --modified`.lines.map(&:strip)
+  end
+
+  # returns an array of relative path names of untracked files, including gitignored files
+  def untracked_files
+    `git ls-files --others`.lines.map(&:strip)
+  end
 end