Remove go mod download step

`go build` will automatically download module dependencies. In many
cases, that is a much smaller set of modules than what is downloaded by
`go mod download`.

Size of GOMODCACHE with `go mod download:

	$ go clean -i -r -cache -modcache
	$ go mod download
	$ du -sh ~/go/pkg/mod
	186M	/home/jun/go/pkg/mod

Size of GOMODCACHE with `go build`:

	$ go clean -i -r -cache -modcache
	$ CGO_ENABLED=0 go build -ldflags="-s -w" cmd/github-mcp-server/main.go
	go: downloading github.com/spf13/viper v1.20.1
	go: downloading github.com/mark3labs/mcp-go v0.18.0
	go: downloading github.com/google/go-github/v69 v69.2.0
	go: downloading github.com/sirupsen/logrus v1.9.3
	go: downloading github.com/spf13/cobra v1.9.1
	go: downloading golang.org/x/sys v0.31.0
	go: downloading github.com/spf13/afero v1.14.0
	go: downloading github.com/fsnotify/fsnotify v1.8.0
	go: downloading github.com/spf13/cast v1.7.1
	go: downloading github.com/go-viper/mapstructure/v2 v2.2.1
	go: downloading github.com/subosito/gotenv v1.6.0
	go: downloading gopkg.in/yaml.v3 v3.0.1
	go: downloading github.com/spf13/pflag v1.0.6
	go: downloading github.com/pelletier/go-toml/v2 v2.2.3
	go: downloading github.com/sagikazarmark/locafero v0.9.0
	go: downloading golang.org/x/text v0.23.0
	go: downloading github.com/google/uuid v1.6.0
	go: downloading github.com/yosida95/uritemplate/v3 v3.0.2
	go: downloading github.com/sourcegraph/conc v0.3.0
	go: downloading github.com/google/go-querystring v1.1.0
	$ du -sh ~/go/pkg/mod
	80M	/home/jun/go/pkg/mod

Reference: https://stackoverflow.com/a/68172023/7902371
Signed-off-by: Eng Zer Jun <[email protected]>

Author: Juneezee

Dockerfile

@@ -8,13 +8,8 @@ WORKDIR /build
 RUN --mount=type=cache,target=/var/cache/apk \
     apk add git
 
-# Install dependencies
-RUN --mount=type=cache,target=/go/pkg/mod \
-    --mount=type=bind,source=go.mod,target=go.mod \
-    --mount=type=bind,source=go.sum,target=go.sum \
-    go mod download
-
 # Build the server
+# go build automatically download required module dependencies to /go/pkg/mod
 RUN --mount=type=cache,target=/go/pkg/mod \
     --mount=type=cache,target=/root/.cache/go-build \
     --mount=type=bind,target=. \