make Dockerfile two-staged

[StefanSchoof]

run in user context
add foreground option

[r00t-]

nice things we can have with --foreground 🎉
(previously one could not run vzlogger inside docker in any sane way, as docker requires the process to stay in foreground.)
(the second stage makes it so that the tools required for compilation don't become part of the image, making for a smaller image. - once #402 is fixed we could switch to alpine for a really small image.)

only thing i would have preferred is to not re-format the unchanged lines, as this now replaces the whole file.

@mbehr1: should we test the Dockerfile as part of the cI checks? can we do that conditionally only if it was changed? or maybe just replace the build with the dockerfile build? questions, questions...

[StefanSchoof]

About the unchanged lines: I know what you mean. Since I removed the sudo and subversion package and extra spaces and the line endings, there are only few unchanged lines. And this format will make changes to packages cleaner. But if you like I can format the apt line as previous.

[r00t-]

imho good to merge,
just i would prefer if i wasn't the only one reviewing stuff.
and all other vzlogger people seem to have little time.

[r00t-]

@StefanSchoof

Since I removed the sudo and subversion package

actually that's probably the reason for noting this, btw.
i stared at the diff for a minute, and could not really tell what was changed.
the best way would probably to first make a commit that only changes the formatting,
and then put the actual change on top of that.
but it's not necessary here.

[m-reuter]

Maybe makes sense to link everything static , also the other dependencies. That way the image will stay smaller as no libs need to be installed.

[StefanSchoof]

I think a complete static version would be great. But this needs work from a person with more knowledge of the c build chain.

[r00t-]

I think a complete[ly] static version would be great

inside docker it doesn't make a difference,
but it would make the plain binary very portable, without needing docker