Build the lightstep image with ubuntu base image.

[arjunrn]

The lightstep image needs libc to load plugins which is missing from the standard skipper Docker image. This PR changes the dockerfile to use a ubuntu base image.

[mikkeloscar]

I would assume it only needs libc because it's being linked against it on the Ubuntu build VM. If we instead build it in a multistage alpine based Dockerfile, then we can keep using the smaller alpine image.

[arjunrn]

@mikkeloscar Already tried something like that, didn't work. I believe that the go plugins need to be build with CGO_ENABLED=1 and then the plugins are loaded by the main skipper binary and this steps needs some(not all) libraries installed. These libraries are not present on the standard alpine image. The other option is to extend the alpine image with only the libraries needed by the skipper binary but I'm not sure about this. @szuecs maybe you have some suggestions.

[szuecs]

let's test it with ubuntu and we can try the approach mikkel suggests to make it better if this works

[szuecs]

👍

[arjunrn]

@mikkeloscar Here's the issue which describes the problem.

[arjunrn]

👍

[mikkeloscar]

@arjunrn @szuecs fine from my side 👍 I might take a look at using alpine, because I'm interested in how the pluginsin go work anyway. :)