Update Golang documentation to use latest stable version (1.13)

Signed-off-by: Jean-Baptiste Le Duigou <[email protected]>

golang/content.md

@@ -13,7 +13,7 @@ Go (a.k.a., Golang) is a programming language first developed at Google. It is a
 The most straightforward way to use this image is to use a Go container as both the build and runtime environment. In your `Dockerfile`, writing something along the lines of the following will compile and run your project:
 
 ```dockerfile
-FROM %%IMAGE%%:1.8
+FROM %%IMAGE%%:1.13
 
 WORKDIR /go/src/app
 COPY . .
@@ -36,13 +36,13 @@ $ docker run -it --rm --name my-running-app my-golang-app
 There may be occasions where it is not appropriate to run your app inside a container. To compile, but not run your app inside the Docker instance, you can write something like:
 
 ```console
-$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.8 go build -v
+$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.13 go build -v
 ```
 
 This will add your current directory as a volume to the container, set the working directory to the volume, and run the command `go build` which will tell go to compile the project in the working directory and output the executable to `myapp`. Alternatively, if you have a `Makefile`, you can run the `make` command inside your container.
 
 ```console
-$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.8 make
+$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.13 make
 ```
 
 ## Cross-compile your app inside the Docker container
@@ -50,13 +50,13 @@ $ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.8 make
 If you need to compile your application for a platform other than `linux/amd64` (such as `windows/386`):
 
 ```console
-$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp -e GOOS=windows -e GOARCH=386 %%IMAGE%%:1.8 go build -v
+$ docker run --rm -v "$PWD":/usr/src/myapp -w /usr/src/myapp -e GOOS=windows -e GOARCH=386 %%IMAGE%%:1.13 go build -v
 ```
 
 Alternatively, you can build for multiple platforms at once:
 
 ```console
-$ docker run --rm -it -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.8 bash
+$ docker run --rm -it -v "$PWD":/usr/src/myapp -w /usr/src/myapp %%IMAGE%%:1.13 bash
 $ for GOOS in darwin linux; do
 >   for GOARCH in 386 amd64; do
 >     export GOOS GOARCH