use COPY instead of ADD, install pkgs then build

- using COPY instead of ADD, since working with local paths, and
    not extracting archived data
  - installing packages before building application binary improves caching,
    and avoids installing packages every time the application code changes

Signed-off-by: Anastas Dancha <anapsix@random.io>

Dockerfile

@@ -1,9 +1,9 @@
 # Build the go application into a binary
 FROM golang:alpine as builder
-WORKDIR /app
-ADD . ./
-RUN CGO_ENABLED=0 GOOS=linux go build -mod vendor -a -installsuffix cgo -o gatus .
 RUN apk --update add ca-certificates
+WORKDIR /app
+COPY . ./
+RUN CGO_ENABLED=0 GOOS=linux go build -mod vendor -a -installsuffix cgo -o gatus .
 
 # Run Tests inside docker image if you don't have a configured go environment
 #RUN apk update && apk add --virtual build-dependencies build-base gcc
@@ -17,4 +17,4 @@ COPY --from=builder /app/static static/
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
 ENV PORT=8080
 EXPOSE ${PORT}
-ENTRYPOINT ["/gatus"]
+ENTRYPOINT ["/gatus"]