use newer ubuntu, cvlc alias, remove double slash, and use correct boundary without --

[bwnance]

this repo has been non-functional as of late, likely due to a vlc update (issue seen here: https://github.com/div-opn/rtsp-to-mjpeg/issues/2 - though the dbus issue wasn't actually the problem)

this PR updates the dockerfile to use alpine:latest, and also cleans up some other things i had problems with:

• // before CAMERASTREAM in script
• boundary shouldn't contain --
• uses cvlc alias instead of initfs --dummy (not needed, but cleaner)