update circleci config to use docker go image

Was using an old ubuntu based machine image, installing Go manually.
This brings it closer in line with the other projects setup, using the
Go docker image.

It also uses the new cimg/go docker image.

.circleci/config.yml

@@ -2,32 +2,22 @@ version: 2.1
 jobs:
   build:
     environment:
-      GO_VERSION: 1.16
       GO111MODULE: "on"
-      CONSUL_VERSION: 1.10.2
-    machine:
-      image: ubuntu-1604:201903-01
-    working_directory: ~/consul-esm
+      CONSUL_VERSION: 1.10.6
+    docker:
+      - image: docker.mirror.hashicorp.services/cimg/go:1.17
     steps:
       - checkout
       - restore_cache:
           keys:
-          - ct-modcache-v2-{{ checksum "go.mod" }}
-      - run: |
-          gopkg="go${GO_VERSION}.linux-amd64.tar.gz"
-          curl -sLo $HOME/go.tar.gz https://dl.google.com/go/$gopkg
-          tar -C $HOME/ -xzf ~/go.tar.gz
+            - ct-modcache-v1-{{ checksum "go.mod" }}
       - run: |
           curl -sLo consul.zip https://releases.hashicorp.com/consul/${CONSUL_VERSION}/consul_${CONSUL_VERSION}_linux_amd64.zip
           unzip consul.zip
           sudo cp consul /usr/local/bin/
       - run: |
-          export GO111MODULE=on
-          export PATH="$HOME/go/bin:$PATH"
-          export GOPATH="$HOME/go"
           make test
       - save_cache:
-          key: ct-modcache-v2-{{ checksum "go.mod" }}
+          key: ct-modcache-v1-{{ checksum "go.mod" }}
           paths:
-          - /home/circleci/go/pkg/mod
-
+            - /go/pkg/mod