/bin/sh: npm: command not found
Your $NODE_VERSION variable has value 6 as defined in the Dockerfile (ENV NODE_VERSION 6) but it should be 6.13.0 so that the following line may work properly:
ENV PATH $NVM_DIR/versions/node/v$NODE_VERSION/bin:$PATH(mapping to /usr/local/nvm/versions/node/v6.13.0/bin)
Otherwise, you are actually generating the following (wrong) path:
/usr/local/nvm/versions/node/v6/bin