chore(rootfs/Dockerfile): update go toolchain to 1.15 (#247)

See https://golang.org/doc/go1.15

Author: mboersma

rootfs/Dockerfile

@@ -5,7 +5,7 @@ LABEL name="deis-go-dev" \
 
 ENV AZCLI_VERSION=2.10.1 \
     DOCKER_VERSION=19.03.4 \
-    GO_VERSION=1.15rc2 \
+    GO_VERSION=1.15 \
     GLIDE_VERSION=v0.13.3 \
     GLIDE_HOME=/root \
     HELM_VERSION=v2.16.9 \