Gitignore all folders beginning with a period

Use one of these patterns:

# ignore all . files but include . folders
.*
!.*/

# ignore all . files and . folders
.*

# Dont ignore .gitignore (this file)
# This is just for verbosity, you can leave it out if
# .gitignore is already tracked or if you use -f to
# force-add it if you just created it
!/.gitignore

# ignore all . folders but include . files
.*/

What is this pattern?

.* - This patter tells git to ignore all the files which starts with .

! - This tells git not to ignore the pattern. In your case /.gitignore

A demo can be found in this answer:
Git: how to ignore hidden files / dot files / files with empty file names via .gitignore?

.*/ will match everything that starts with a dot and is a folder

With the following command you can test it: mkdir test && cd test && git init && mkdir -p .foo .foo/.bar foo/.bar && touch .foo/dummy .foo/.bar/dummy foo/.bar/dummy && git add . && git status && echo '.*/'>.gitignore && git reset && git add . && git status