ci: also build the interop image on pull requests, but skip pushing (#5115)

* ci: also build the interop image on pull requests, but skip pushing

* use checked out code for interop Docker build

interop/Dockerfile

@@ -3,7 +3,7 @@ FROM martenseemann/quic-network-simulator-endpoint:latest AS builder
 ARG TARGETPLATFORM
 RUN echo "TARGETPLATFORM: ${TARGETPLATFORM}"
 
-RUN apt-get update && apt-get install -y wget tar git
+RUN apt-get update && apt-get install -y wget tar git && rm -rf /var/lib/apt/lists/*
 
 ENV GOVERSION=1.24.0
 
@@ -18,13 +18,8 @@ ENV PATH="/go/bin:${PATH}"
 # build with --build-arg CACHEBUST=$(date +%s)
 ARG CACHEBUST=1
 
-# build other branches / commits / tags using --build-arg GITREF="<git reference>"
-ARG GITREF="master"
-
-RUN git clone https://github.com/quic-go/quic-go
+COPY . /quic-go
 WORKDIR /quic-go
-RUN git checkout ${GITREF}
-RUN go get ./...
 
 RUN git rev-parse HEAD | tee commit.txt
 RUN go build -o server -ldflags="-X github.com/quic-go/quic-go/qlog.quicGoVersion=$(git describe --always --long --dirty)" interop/server/main.go
@@ -36,8 +31,8 @@ FROM martenseemann/quic-network-simulator-endpoint:latest
 WORKDIR /quic-go
 
 COPY --from=builder /quic-go/commit.txt /quic-go/server /quic-go/client ./
+COPY --from=builder /quic-go/interop/run_endpoint.sh ./
 
-COPY run_endpoint.sh .
 RUN chmod +x run_endpoint.sh
 
 ENTRYPOINT [ "./run_endpoint.sh" ]