docs: Update README for bootstrap node docker

[Tha14]

Old instructions do not work as we do not tag every version, we only tag latest version for the image.

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This change is 

[JFreegman]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 1 approvals obtained

[codecov]

Codecov Report

Merging #2333 (fb99aea) into master (0a277b5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2333   +/-   ##
=======================================
  Coverage   78.15%   78.15%           
=======================================
  Files         140      140           
  Lines       31045    31045           
=======================================
  Hits        24264    24264           
  Misses       6781     6781           

Impacted Files	Coverage Δ
toxcore/TCP_client.c	83.37% <0.00%> (-0.71%)	⬇️
toxcore/group_chats.c	69.31% <0.00%> (-0.26%)	⬇️
toxcore/group.c	84.41% <0.00%> (-0.07%)	⬇️
toxcore/TCP_server.c	78.58% <0.00%> (ø)
toxcore/net_crypto.c	87.43% <0.00%> (+0.14%)	⬆️
auto_tests/group_sync_test.c	85.11% <0.00%> (+0.46%)	⬆️
toxcore/friend_connection.c	85.41% <0.00%> (+0.69%)	⬆️
auto_tests/group_invite_test.c	99.22% <0.00%> (+0.77%)	⬆️
toxcore/ping.c	87.03% <0.00%> (+1.23%)	⬆️
toxav/groupav.c	75.91% <0.00%> (+1.33%)	⬆️

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