Documentation: Add note about pinning the container after release #2566

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Job	Run time
linux rpm fedora	7m 9s
Prepare container	2m 3s
dpdk gcc	5s
osx clang --disable-ssl	4m 38s
linux gcc --disable-ssl	2m 26s
linux clang --disable-ssl	2m 30s
linux gcc test -500	14m 24s
linux gcc test 501-1000	9m 54s
linux gcc test 1001-	5m 15s
linux clang test sanitizers -300	11m 11s
linux clang test sanitizers 301-600	14m 16s
linux clang test sanitizers 601-900	9m 8s
linux clang test sanitizers 901-1200	7m 46s
linux clang test sanitizers 1201-	6m 29s
linux gcc test -ljemalloc -500	15m 14s
linux gcc test -ljemalloc 501-1000	10m 33s
linux gcc test -ljemalloc 1001-	5m 25s
linux gcc system-test-dpdk dpdk -100	11m 18s
linux gcc system-test-dpdk dpdk 101-200	15m 21s
linux gcc system-test-dpdk dpdk 201-	21m 58s
linux gcc system-test-userspace -100	10m 1s
linux gcc system-test-userspace 101-200	13m 44s
linux gcc system-test-userspace 201-	20m 26s
linux gcc system-test -100	13m 1s
linux gcc system-test 101-200	16m 13s
linux gcc system-test 201-	23m 36s
linux clang system-test sanitizers -100	14m 42s
linux clang system-test sanitizers 101-200	17m 51s
linux clang system-test sanitizers 201-	25m 35s
linux x86 gcc --disable-ssl	2m 26s
	5h 34m 38s

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