🗂️ refactor: organize Dockerfiles into docker/ directory and update all references

docker/Dockerfile.build

@@ -0,0 +1,38 @@
+# Build environment Dockerfile with pre-installed Go tools and dependencies
+# Optimized for CI/CD pipeline speed
+
+FROM golang:1.26.1-alpine AS builder
+
+# Install build dependencies
+RUN apk add --no-cache \
+    git \
+    bash \
+    curl \
+    make \
+    gcc \
+    musl-dev \
+    bc \
+    grep \
+    sed \
+    jq \
+    ca-certificates
+
+# Set up Go environment
+ENV GOPATH=/go
+ENV PATH=$GOPATH/bin:/usr/local/go/bin:/usr/local/bin:/usr/bin:/bin
+WORKDIR /go/src/dance-lessons-coach
+
+# Install common Go tools
+RUN go install github.com/swaggo/swag/cmd/swag@latest && \
+    go install golang.org/x/tools/cmd/goimports@latest && \
+    go install honnef.co/go/tools/cmd/staticcheck@latest
+
+# Copy only go.mod and go.sum first for dependency caching
+COPY go.mod go.sum ./
+RUN go mod download && go mod verify
+
+# Simple build environment - source code is mounted at runtime
+WORKDIR /workspace
+
+# Pre-download common Go tools (already installed in base)
+# RUN go install github.com/swaggo/swag/cmd/swag@latest