Merge workflows into one CI pipeline

- Unit tests, integration tests and the Docker build check run in
  parallel on branches and pull requests
- Integration tests run on every push instead of only manually
- Branch builds only build amd64 and check the arm64 cross-compile in the
  test job; both platforms are built and pushed only for version tags,
  after all tests pass
- Docker layers cached through the runner cache, Go module and build
  caches mounted in the Dockerfile
- A newer push to the same branch cancels the running pipeline

Dockerfile

@@ -7,11 +7,13 @@ FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS build
 ARG TARGETOS TARGETARCH
 WORKDIR /app
 COPY go.mod go.sum ./
-RUN go mod download
+RUN --mount=type=cache,target=/go/pkg/mod go mod download
 COPY cmd/ ./cmd/
 COPY internal/ ./internal/
-RUN go tool templ generate
-RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
+RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build \
+    go tool templ generate
+RUN --mount=type=cache,target=/go/pkg/mod --mount=type=cache,target=/root/.cache/go-build \
+    CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
     go build -trimpath -ldflags="-s -w" -tags timetzdata -o main ./cmd
 
 # Stage 2: Frontend static files (the same for every platform)