Make Docker image easier to use (#50)

* Use single server example as default frontend

This just works out of the box

* Refactor Dockerfile

- Remove the asset dir since it's embedded in the executable anyway
- Use the code in the current directory to build instead of downloading.
  Makes local testing very easy.
- Cache Go dependencies
- Run without root privileges. Note that using a BoltDB database
  requires mounting a volume with the correct permissions. The default
  database has thus been changed to memory so that starting the image
  just works.

* Automatically publish images

Dockerfile

@@ -1,20 +1,18 @@
-FROM golang:alpine AS build_base
-#ENV GOARCH arm64
-#ENV GOARCH amd64
-RUN apk add --no-cache git gcc ca-certificates libc-dev \
-&& mkdir -p /go/src/github.com/librespeed/ \
-&& cd /go/src/github.com/librespeed/ \
-&& git clone https://github.com/librespeed/speedtest-go.git
-WORKDIR /go/src/github.com/librespeed/speedtest-go
-RUN go get ./ && go build -ldflags "-w -s" -trimpath -o speedtest main.go
+FROM golang:1.18-alpine AS build_base
+RUN apk add --no-cache git gcc ca-certificates libc-dev
+WORKDIR /build
+COPY go.mod go.sum ./
+RUN go mod download
+COPY ./ ./
+RUN go build -ldflags "-w -s" -trimpath -o speedtest .
 
-FROM alpine:3.15
-RUN apk add ca-certificates
+FROM alpine:3.16
+RUN apk add --no-cache ca-certificates
 WORKDIR /app
-COPY --from=build_base /go/src/github.com/librespeed/speedtest-go/speedtest .
-COPY --from=build_base /go/src/github.com/librespeed/speedtest-go/web/assets ./assets
-COPY --from=build_base /go/src/github.com/librespeed/speedtest-go/settings.toml .
+COPY --from=build_base /build/speedtest ./
+COPY settings.toml ./
 
+USER nobody
 EXPOSE 8989
 
 CMD ["./speedtest"]