Update golang Docker tag to v1.21.4

Dockerfile

-FROM golang:1.21.0 as build
+FROM golang:1.21.4 as build
 ADD . /src
 RUN cd /src && \
     go build -ldflags="-extldflags=-static" -tags "sqlite_omit_load_extension netgo" -o assetmon ./cmd/assetmon && \