enable git commit signing via a ssh key

gitconfig.erb

@@ -1,6 +1,7 @@
 [user]
-	name = <%= print("Your Name: "); STDOUT.flush; STDIN.gets.chomp %>
-	email = <%= print("Your Email: "); STDOUT.flush; STDIN.gets.chomp %>
+  name = <%= print("Your Name: "); STDOUT.flush; STDIN.gets.chomp %>
+  email = <%= print("Your Email: "); STDOUT.flush; STDIN.gets.chomp %>
+  signingkey = <%= ENV['HOME'] %>/.ssh/signing-key.pub
 [alias]
   co = checkout
   tagcommit = !sh -c 'git rev-list -1 $0'
@@ -12,21 +13,25 @@
   # https://stackoverflow.com/a/35075021/845546
   adog = log --all --decorate --oneline --graph
 [color]
-	diff = auto
-	status = auto
-	branch = auto
+  diff = auto
+  status = auto
+  branch = auto
 [core]
-	excludesfile = <%= ENV['HOME'] %>/.gitignore
-	autocrlf = input
+  excludesfile = <%= ENV['HOME'] %>/.gitignore
+  autocrlf = input
 [apply]
-	whitespace = nowarn
+  whitespace = nowarn
 [format]
-	pretty = %C(yellow)%h%Creset %s %C(red)(%an, %cr)%Creset
+  pretty = %C(yellow)%h%Creset %s %C(red)(%an, %cr)%Creset
 [github]
-	user = <%= print("GitHub Username: "); STDOUT.flush; STDIN.gets.chomp %>
-	token = <%= print("GitHub API Token: "); STDOUT.flush; STDIN.gets.chomp %>
+  user = <%= print("GitHub Username: "); STDOUT.flush; STDIN.gets.chomp %>
+  token = <%= print("GitHub API Token: "); STDOUT.flush; STDIN.gets.chomp %>
 [push]
   default = simple
 [status]
   short = true
   branch = true
+[gpg]
+  format = ssh
+[commit]
+  gpgsign = true