docker: Accept Go version as --build-arg

Dockerfile

@@ -1,4 +1,5 @@
-FROM golang:1.13 AS builder
+ARG GOVERSION=latest
+FROM golang:$GOVERSION AS builder
 
 WORKDIR /src
 COPY . .

Dockerfile.builder

@@ -1,4 +1,5 @@
-FROM golang:1.13
+ARG GOVERSION=latest
+FROM golang:$GOVERSION
 
 # FPM to build Debian packages
 RUN apt-get update && apt-get install -y --no-install-recommends \

Dockerfile.stdiscosrv

@@ -1,4 +1,5 @@
-FROM golang:1.13 AS builder
+ARG GOVERSION=latest
+FROM golang:$GOVERSION AS builder
 
 WORKDIR /src
 COPY . .

Dockerfile.strelaysrv

@@ -1,4 +1,5 @@
-FROM golang:1.13 AS builder
+ARG GOVERSION=latest
+FROM golang:$GOVERSION AS builder
 
 WORKDIR /src
 COPY . .