deps: upgrade to use go1.18

Signed-off-by: Rui Chen <rui@chenrui.dev>

.travis.yml

@@ -1,6 +1,6 @@
 language: go
 go:
-- "1.17.x"
+- "1.18.x"
 
 notifications:
   email: false

Dockerfile

@@ -1,5 +1,5 @@
 # build
-FROM golang:1.17.5-stretch AS build-env
+FROM golang:1.18-stretch AS build-env
 
 WORKDIR /src
 
@@ -8,7 +8,7 @@ ADD . .
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -o app .
 
 # run
-FROM alpine:3.15
+FROM alpine:3.16
 
 RUN apk add --no-cache \
     ca-certificates

go.mod

@@ -1,6 +1,6 @@
 module github.com/gotify/cli/v2
 
-go 1.17
+go 1.18
 
 require (
 	github.com/adrg/xdg v0.2.1