x86_64: Clean cache after apt-get install.

Signed-off-by: DL6ER <dl6er@dl6er.de>

docker/x86_64/Dockerfile

@@ -3,11 +3,11 @@ FROM debian:stretch
 RUN apt-get update && \
     apt-get install -y --no-install-recommends nettle-dev gcc libc-dev \
         make file wget netcat-traditional sqlite3 git ca-certificates ssh libcap-dev \
-        dnsutils libcap2-bin
+        dnsutils libcap2-bin && rm -rf /var/lib/apt/lists/*
 
 # Install ghr for GitHub Releases: https://github.com/tcnksm/ghr
 RUN wget https://github.com/tcnksm/ghr/releases/download/v0.12.0/ghr_v0.12.0_linux_amd64.tar.gz && \
-    tar -xzf ghr_*_linux_amd64.tar.gz && \
+    tar -xzf ghr_*_linux_amd64.tar.gz && rm ghr_*_linux_amd64.tar.gz && \
     mv ghr_*_linux_amd64/ghr /usr/bin/ghr
 
 ENV CC gcc