Teamcity Sencha NPM Login Method. This Generates the ~/.npmrc login credentials.

teamcity-buildstep.sh

# Build step
# You can replicate this by running `npm login --registry=https://npm.sencha.com --scope=@sencha` on your system. 
# Get your authtoken by logging in on your system and finding the auth token in ~/.npmrc.

# Remove previous file
rm -f ~/.npmrc

# Write new ~/.npmrc
echo "Writing auth tokens to ~/.npmrc"
echo "@sencha:registry=http://npm.sencha.com" >> ~/.npmrc
echo "//npm.sencha.com/:_authToken=%npm.sencha.com-authtoken%" >> ~/.npmrc

# Debug
echo "Debug contents of ~/.npmrc:"
echo "~~~~~~~~~~~~~~~~~~~~~~"
cat ~/.npmrc
echo "~~~~~~~~~~~~~~~~~~~~~~"

# Check login with whoami
echo "Invoking NPM whoami"
echo "~~~~~~~~~~~~~~~~~~~~~~"
npm --registry http://npm.sencha.com/ whoami
echo "~~~~~~~~~~~~~~~~~~~~~~"

TeamCity Variable: %npm.sencha.com-authtoken% - It doesn't have to be a variable.
Replace: %npm.sencha.com-authtoken% with your token from the account you want to use. I copy the token from my local ~/.npmrc file.