1 - Generate private key:

dropbearkey -t rsa -s 4096 -f ~/.ssh/id_dropbear

2 - Calculate public key:

dropbearkey -y -f ~/.ssh/id_dropbear | grep '^ssh-rsa ' > ~/.ssh/id_dropbear.pub

3 - Create ‘wrapper’ for ssh (be careful of it’s quote):

echo 'ssh -y -i ~/.ssh/id_dropbear $*' > ~/../usr/bin/ssh-id
chmod +x ~/../usr/bin/ssh-id

4 - Add this line into ~/.bashrc:

export GIT_SSH=$HOME/../usr/bin/ssh-id

5 - Add SSH public key from step 2 to Github SSH Keys

6 - Replace remote URL

git remote remove origin
git remote add origin git@github.com:<username>/<repo>.git

7 - Push

git push -u origin master

When pushing next time, you can simply use:

git push

References: