Update Dockerfile to expose 8080

[cbarange]

Hello I saw that 8080 wasn't expose in the dockerfile. So just fix it

[benzino77]

Hmmm. The port on which CRA will listen to is defined by ENV variable: APP_PORT and the default is (as I remember) 3000. I'm not sure it will be valid to include EXPOSE clause in the Dockerfile as the listen/expose port can be changed by the one who start the container. Have a look also here: https://docs.docker.com/engine/reference/builder/#expose - it looks like EXPOSE is a kind of "documentation"