Switch golang base image to Debian in order to address libc linking p…

[cstsw]

…roblem

Alpine has subistituted GNU libc with musl in 2014
(see https://www.alpinelinux.org/posts/Alpine-Linux-has-switched-to-musl-libc.html)

So if one builds ssl-proxy on Alpine, the linux binary will contain a reference to
the musl library. Since this library is not present in major linux distributions
(like Debian), launching ssl-proxy results in a "No such file or directory" error.
For more details on this problem see e.g.
https://www.reddit.com/r/golang/comments/64lz0s/how_to_crosscompile_libc_variants_like_musl/
https://stackoverflow.com/questions/54524785/how-to-build-a-go-executable-that-doesnt-link-to-musl-libc

Therefore the golang base image is switched from the Alpine to the Debian variant,
using it's most recent image of the 1.13 version line.

[cstsw]

I would suggest switching the golang base image from Alpine to Debian in order to resolve #38