fix docker image to worj caddy 2.8

Author: alancolant

Dockerfile

@@ -1,12 +1,12 @@
-FROM caddy:builder-alpine AS builder
+FROM caddy:2.8-builder-alpine AS builder
 
 RUN apk add --update --no-cache make vips-dev gcc musl-dev
 
 ADD . .
 RUN make build
 
 # FINAL IMAGE
-FROM caddy:alpine
+FROM caddy:2.8-alpine
 RUN apk add --update --no-cache vips
 
 COPY --from=builder /usr/bin/out/caddy /usr/bin/caddy