Upgrade Go from 1.13 to 1.15

Makefile

@@ -28,7 +28,7 @@ SUBMODULES=_submodules
 UID:=$(shell id -u)
 GID:=$(shell id -g)
 
-FIRECRACKER_CONTAINERD_BUILDER_IMAGE?=golang:1.13-buster
+FIRECRACKER_CONTAINERD_BUILDER_IMAGE?=golang:1.15-buster
 export FIRECRACKER_CONTAINERD_TEST_IMAGE?=localhost/firecracker-containerd-test
 export GO_CACHE_VOLUME_NAME?=gocache
 

tools/docker/Dockerfile.integ-test

@@ -1,7 +1,7 @@
 # syntax=docker/dockerfile:experimental
 # Test image that starts up containerd and the devmapper snapshotter. The default CMD will drop to a bash shell. Overrides
 # to CMD will be provided appended to /bin/bash -c
-FROM golang:1.13-stretch
+FROM golang:1.15-stretch
 ENV PATH="/bin:/usr/bin:/usr/local/bin:/sbin:/usr/sbin:/usr/local/sbin:/usr/local/go/bin"
 ENV INSTALLROOT="/usr/local"
 ENV DEBIAN_FRONTEND="noninteractive"

tools/docker/Dockerfile.proto-builder

@@ -11,7 +11,7 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-FROM golang:1.13-stretch
+FROM golang:1.15-stretch
 
 RUN apt-get update && apt-get install --yes --no-install-recommends \
 	libprotobuf-dev \

tools/docker/Dockerfile.runc-builder

@@ -11,7 +11,7 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-FROM golang:1.13-stretch
+FROM golang:1.15-stretch
 
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get -y install libseccomp-dev pkg-config