Fix Build (go-smtp 0.19)

[JZL]

Hi,

I've been updating my server and realized that the most recent few versions of mailcatcher have been failing the build. I don't really know golang so this is just me messing around but I got the build green and it works in my quick tests.

Comments for each commit:

• 824bed: Just adding gcc was causing the issue for the builds:

fatal error: stdlib.h: No such file or directory

I didn't really figure out why but build-base works and only makes the builder docker image slightly larger.

• Rest: I just went off the compiler errors and copied from the smtp-debug-server, working up through each major version: https://github.com/emersion/go-smtp/blob/v0.19.0/cmd/smtp-debug-server/main.go.

I'm not that experienced with github actions but to test the build I had to remove the pushing part from the workflow. You can see here that each commit has a happy green build.

Happy Holidays!
Jonah