Fix Dockerfile

[StefMa]

If I build with docker like

docker build -t bore  .                                                                                                                             

it reports an error:

1 warning found (use docker --debug to expand):

• FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 1)

Seems as should be in uppercase (or both in lowecase) 🙃
See also https://docs.docker.com/reference/build-checks/from-as-casing/

[sagar676]

is bore.pub giving timeout?

[StefMa]

I don't know 🤷‍♂️ but this change hasn't anything to do with the problem mentioned in #150

[ekzhang]

Thanks! I'm glad they're clamping down on casing, haha. Have been doing FROM image:tag as name for so long that I thought that was actually the official syntax.