Update Go to 1.18

This PR updates the `go.mod`, Docker image, and CI workflows to use Go
1.18, the latest version of Go.

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Set up Go 1.x
         uses: actions/setup-go@v2
         with:
-          go-version: ^1.17
+          go-version: ^1.18
 
       - name: Check out code into the Go module directory
         uses: actions/checkout@v2
@@ -34,4 +34,4 @@ jobs:
         run: make get
 
       - name: Test
-        run: make test
+        run: make test

Dockerfile

@@ -1,4 +1,4 @@
-ARG GOLANG_VERSION=1.17
+ARG GOLANG_VERSION=1.18
 FROM golang:${GOLANG_VERSION}-buster as builder
 
 ARG VIPS_VERSION=8.12.2
@@ -72,4 +72,4 @@ USER nobody
 
 ENTRYPOINT ["/usr/local/bin/imagor"]
 
-EXPOSE ${PORT}
+EXPOSE ${PORT}

go.mod

@@ -1,6 +1,6 @@
 module github.com/cshum/imagor
 
-go 1.17
+go 1.18
 
 require (
 	cloud.google.com/go/storage v1.21.0