Supplying a blank tag generates an invalid config

[eahlberg]

Expected behaviour

I expect the config to be valid even though I do not supply a tag. If the tag really is required (I don't know if that's the case), it'd be helpful if a) it was stated or even better if b) the CLI forced me to enter a tag.

Steps to reproduce

Run the following in the dockerfile-wizard repo:

1. make ready
2. make setup
3. Choose a name for the project, leave the tag blank and choose a linux version. E.g. choose name: foo, linux version: 4 and leave the rest of the options blank.
4. Validate the config file and it will have an error, e.g. on the line which containsimage: $DOCKER_USERNAME/foo:.

FYI I'm using macOS Mojave.

Possible fix

Update the config template to not include a colon between name and tag. Only append a tag prepended with a colon when a tag is supplied.