Update go version (#355)

* Automatically get the latest go version when building the image

* Update cron from Monday to Thursday (seems like go releaes
tuesdays/wednesdays)

.github/workflows/build-buildpacks-ci-image.yml

@@ -3,7 +3,7 @@ name: Build buildpacks-ci docker image
 on:
   workflow_dispatch: { }
   schedule:
-    - cron: "0 1 * * 1"
+    - cron: "0 1 * * 4"
   push:
     branches: [ master ]
     paths: [ Dockerfile, config/**, build/** , Gemfile, Gemfile.lock ]

Dockerfile

@@ -152,6 +152,10 @@ RUN cd /usr/local \
   && tar xf go.tar.gz \
   && rm go.tar.gz
 
+RUN export GO_VERSION=$(wget -qO- https://golang.org/dl/?mode=json | grep -oP '"version": "\K([^"]+)' | head -n 1) && \
+    wget -q https://golang.org/dl/go$GO_VERSION.linux-amd64.tar.gz && \
+    tar -C /usr/local -xzf go$GO_VERSION.linux-amd64.tar.gz
+
 ENV GOROOT=/usr/local/go
 ENV GOPATH=/go
 ENV PATH=$GOPATH/bin:$GOROOT/bin:$PATH