Clarify heap setting in Docker docs (#42885)
Clarify heap setting in Docker docs (#42885)

Add note in the Docker docs that even when container memory is limited,
we still require specifying -Xms/-Xmx using one of the supported
methods.

Backport of #42754 
vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,vroyer/elassandra