ci: optimize pipeline with image retagging and conditional tests

Dockerfile

@@ -4,11 +4,17 @@
 FROM node:22-alpine AS frontend-builder
 WORKDIR /app/frontend
 COPY frontend/package*.json ./
-RUN npm install --no-audit --no-fund
+RUN --mount=type=cache,target=/root/.npm npm install --no-audit --no-fund
 COPY frontend/ ./
 # Version shown in the header: the git tag for releases (set by CI), otherwise "<package version>-dev"
 ARG APP_VERSION=""
-RUN npx tsc --noEmit && npm run lint && npm run test:run && APP_VERSION="${APP_VERSION}" npm run build
+ARG SKIP_TESTS=0
+RUN if [ "$SKIP_TESTS" != "1" ]; then \
+      npx tsc --noEmit && npm run lint && npm run test:run; \
+    else \
+      echo "Skipping frontend tests (SKIP_TESTS=1)"; \
+    fi && \
+    APP_VERSION="${APP_VERSION}" npm run build
 
 # ==============================================================================
 # Stage 2: Go Backend Lint, Unit Tests & Binary Build (Cached Go module & sqlite layer)
@@ -19,7 +25,8 @@ RUN apk add --no-cache git ca-certificates
 
 # Cache Go modules & pre-compile heavy dependencies (modernc.org/sqlite) in a dedicated layer
 COPY go.mod ./
-RUN go get github.com/go-chi/chi/v5@v5.2.0 \
+RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build \
+    go get github.com/go-chi/chi/v5@v5.2.0 \
     github.com/go-chi/cors@v1.2.1 \
     github.com/golang-jwt/jwt/v5@v5.2.1 \
     github.com/gorilla/websocket@v1.5.3 \
@@ -38,10 +45,16 @@ COPY cmd/ ./cmd/
 COPY internal/ ./internal/
 COPY --from=frontend-builder /app/frontend/dist ./cmd/server/dist
 
+ARG SKIP_TESTS=0
 # Run go mod tidy, static analysis (go vet), unit tests (go test), and compile final binary
-RUN go mod tidy && \
-    CGO_ENABLED=0 go vet ./... && \
-    CGO_ENABLED=0 go test -v ./... && \
+RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build \
+    go mod tidy && \
+    if [ "$SKIP_TESTS" != "1" ]; then \
+      CGO_ENABLED=0 go vet ./... && \
+      CGO_ENABLED=0 go test -v ./...; \
+    else \
+      echo "Skipping backend tests (SKIP_TESTS=1)"; \
+    fi && \
     CGO_ENABLED=0 go build -ldflags="-s -w" -o /app/mindsift ./cmd/server
 
 # ==============================================================================