kernel: Use local kernel source if available

The kernel build currently downloads the source tar ball every
time, which is a little tedious when experimenting with kernel
configs or when compiling the kernel multiple times.

This commit adds a new 'fetch' make target which downloads the
kernel sources into ./sources. Then in the Dockerfile we add
the directory and only download the source if it is not present.

The tarballs signature is till checked on each build.

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

kernel/Dockerfile

@@ -46,11 +46,12 @@ ENV WIREGUARD_URL=https://git.zx2c4.com/WireGuard/snapshot/WireGuard-${WIREGUARD
 COPY keys.asc keys.asc
 
 # Download and verify kernel
+COPY sources/ /
 RUN curl -fsSLO ${KERNEL_SHA256_SUMS} && \
     gpg2 -q --import keys.asc && \
     gpg2 --verify sha256sums.asc && \
     KERNEL_SHA256=$(grep linux-${KERNEL_VERSION}.tar.xz sha256sums.asc | cut -d ' ' -f 1) && \
-    curl -fsSLO ${KERNEL_SOURCE} && \
+    [ -f linux-${KERNEL_VERSION}.tar.xz ] || curl -fsSLO ${KERNEL_SOURCE} && \
     echo "${KERNEL_SHA256}  linux-${KERNEL_VERSION}.tar.xz" | sha256sum -c - && \
     xz -d linux-${KERNEL_VERSION}.tar.xz && \
     curl -fsSLO ${KERNEL_PGP2_SIGN} && \