Separate build and push actions into individual functions

[Ana Perez Ghiglia]

Define separate build and push functions so

• we can receive and pass extra parameters to the docker build and docker push commands (i.e.: --build-arg)
• the user can override only one of them but still leverage of the definition of dockerBuildAndPush

Having this will help fixing #2 - the user will be able to invoke only build function for non "pushable" branches if desired

[Julien Portalier]

Let's note that #16 implements the -o option to pass build options such as --build-arg to the docker build command.

But being able to build & push independently would be nice.