Update golang Docker tag to v1.18

Dockerfile

@@ -5,7 +5,7 @@ RUN npm install && env NODE_OPTIONS=--openssl-legacy-provider ./node_modules/.bi
 
 # Debian bullseye can't build gostatic due to its strict dependency
 # on Go 1.17, so we build it separately.
-FROM golang:1.17 AS gobuild
+FROM golang:1.18 AS gobuild
 RUN go get github.com/piranha/gostatic
 RUN go get git.autistici.org/ai/webtools/cmd/sitesearch
 RUN go get git.autistici.org/ai/webtools/cmd/jsonsubst