fix: Docker build

Author: ViRb3

Dockerfile

@@ -3,5 +3,6 @@ FROM golang:1.18.1-alpine AS builder
 WORKDIR /src
 COPY . .
 
-RUN go mod download && \
+RUN apk add --no-cache git && \
+    go mod download && \
     CGO_ENABLED=0 go build -ldflags="-s -w" -o "SignTools-Builder"