Bump Golang 1.12.4

go1.12.4 (released 2019/04/11) fixes an issue where using the prebuilt
binary releases on older versions of GNU/Linux led to failures when linking
programs that used cgo. Only Linux users who hit this issue need to update.

See golang/go#31293 for details

Full diff: golang/go@go1.12.3...go1.12.4

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -1,4 +1,4 @@
-FROM    golang:1.12.3
+FROM    golang:1.12.4
 
 # allow replacing httpredir or deb mirror
 ARG     APT_MIRROR=deb.debian.org