fix(backend): simplify Go build command in Dockerfile

Change build command from `go build -o server ./main.go` to `go build -o server .` to build from the current directory instead of specifying main.go explicitly.

backend/Dockerfile

@@ -1,7 +1,7 @@
 FROM golang:1.22-alpine AS build
 WORKDIR /app
 COPY . .
-RUN go build -o server ./main.go
+RUN go build -o server .
 
 FROM alpine:3.20
 WORKDIR /app