Documentation: Add note about pinning the container after release #2567

# to view logs

Job	Run time
linux rpm fedora	6m 59s
osx clang --disable-ssl	5m 3s
Prepare container	2m 6s
linux gcc --disable-ssl	2m 17s
linux clang --disable-ssl	2m 19s
linux gcc test -500	13m 50s
linux gcc test 501-1000	9m 46s
linux gcc test 1001-	4m 38s
linux clang test sanitizers -300	11m 17s
linux clang test sanitizers 301-600	14m 18s
linux clang test sanitizers 601-900	8m 47s
linux clang test sanitizers 901-1200	6m 36s
linux clang test sanitizers 1201-	6m 9s
linux gcc test -ljemalloc -500	14m 40s
linux gcc test -ljemalloc 501-1000	9m 55s
linux gcc test -ljemalloc 1001-	4m 54s
linux gcc system-test-userspace -100	10m 4s
linux gcc system-test-userspace 101-200	13m 45s
linux gcc system-test-userspace 201-	18m 1s
linux gcc system-test -100	13m 5s
linux gcc system-test 101-200	15m 31s
linux gcc system-test 201-	23m 9s
linux clang system-test sanitizers -100	14m 47s
linux clang system-test sanitizers 101-200	17m 21s
linux clang system-test sanitizers 201-	25m 14s
linux x86 gcc --disable-ssl	2m 13s
	4h 36m 44s

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