unix/linux: update Dockerfile to Kernel 5.14

Change-Id: Ie6b9521c70c0e91a507a923811f44f797b64e064
Reviewed-on: https://go-review.googlesource.com/c/sys/+/346029
Trust: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Matt Layher <[email protected]>

Author: tklauser

unix/linux/Dockerfile

@@ -15,8 +15,8 @@ RUN apt-get update && apt-get install -y  --no-install-recommends \
 # Get the git sources. If not cached, this takes O(5 minutes).
 WORKDIR /git
 RUN git config --global advice.detachedHead false
-# Linux Kernel: Released 27 Jun 2021
-RUN git clone --branch v5.13 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
+# Linux Kernel: Released 29 Aug 2021
+RUN git clone --branch v5.14 --depth 1 https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
 # GNU C library: Released 02 Aug 2021
 RUN git clone --branch release/2.34/master --depth 1 https://sourceware.org/git/glibc.git