Small fixes (#32)

* fix docker build warning

prior to this, building the image was causing this warning:

```
 1 warning found (use docker --debug to expand):
 - FromAsCasing: 'as' and 'FROM' keywords' casing do not match (line 2)
```

* dockerfiles: cache go mod dependencies in their own layer

this change causes docker to create an additional intermediate layer containing
just the go.mod file and the downloaded dependencies cited therefrom. this
prevents having to re-download the go.mod dependencies on EVERY image rebuild.
more specifically, as long as you don't change the go.mod file, docker will use
the cached layer.

* README: dev dependencies: add gci

it's used in the Makefile but, prior to this, the README didn't instruct you to
install it.

README.md

@@ -45,6 +45,7 @@ go install mvdan.cc/[email protected]
 go install honnef.co/go/tools/cmd/[email protected]
 go install github.com/golangci/golangci-lint/cmd/[email protected]
 go install go.uber.org/nilaway/cmd/[email protected]
+go install github.com/daixiang0/[email protected]
 ```
 
 **Lint, test, format**

cli.dockerfile

@@ -1,7 +1,10 @@
 # syntax=docker/dockerfile:1
-FROM golang:1.21 as builder
+FROM golang:1.21 AS builder
 ARG VERSION
 WORKDIR /build
+ADD go.mod /build/
+RUN --mount=type=cache,target=/root/.cache/go-build CGO_ENABLED=0 GOOS=linux \
+       go mod download
 ADD . /build/
 RUN --mount=type=cache,target=/root/.cache/go-build CGO_ENABLED=0 GOOS=linux \
     go build \

httpserver.dockerfile

@@ -1,7 +1,10 @@
 # syntax=docker/dockerfile:1
-FROM golang:1.21 as builder
+FROM golang:1.21 AS builder
 ARG VERSION
 WORKDIR /build
+ADD go.mod /build/
+RUN --mount=type=cache,target=/root/.cache/go-build CGO_ENABLED=0 GOOS=linux \
+       go mod download
 ADD . /build/
 RUN --mount=type=cache,target=/root/.cache/go-build CGO_ENABLED=0 GOOS=linux \
     go build \