Straightforward set to 512MB doesn't work:
$ docker run --rm -e "ES_JAVA_OPTS=-Xms512m -Xms512m" docker.elastic.co/elasticsearch/elasticsearch:6.6.0
OpenJDK 64-Bit Server VM warning: Option UseConcMarkSweepGC was deprecated in version 9.0 and will likely be removed in a future release.
[2019-05-17T01:43:17,857][INFO ][o.e.e.NodeEnvironment ] [Ab_wy_n] using [1] data paths, mounts [[/ (overlay)]], net usable_space [9.1gb], net total_space [87.7gb], types [overlay]
[2019-05-17T01:43:17,860][INFO ][o.e.e.NodeEnvironment ] [Ab_wy_n] heap size [1015.6mb], compressed ordinary object pointers [true]
...
[2019-05-17T01:43:17,871][INFO ][o.e.n.Node ] [Ab_wy_n] JVM arguments [-Xms1g, -Xmx1g, -XX:+UseConcMarkSweepGC, -XX:CMSInitiatingOccupancyFraction=75, -XX:+UseCMSInitiatingOccupancyOnly, -Des.networkaddress.cache.ttl=60, -Des.networkaddress.cache.negative.ttl=10, -XX:+AlwaysPreTouch, -Xss1m, -Djava.awt.headless=true, -Dfile.encoding=UTF-8, -Djna.nosys=true, -XX:-OmitStackTraceInFastThrow, -Dio.netty.noUnsafe=true, -Dio.netty.noKeySetOptimization=true, -Dio.netty.recycler.maxCapacityPerThread=0, -Dlog4j.shutdownHookEnabled=false, -Dlog4j2.disable.jmx=true, -Djava.io.tmpdir=/tmp/elasticsearch-4988010857367449349, -XX:+HeapDumpOnOutOfMemoryError, -XX:HeapDumpPath=data, -XX:ErrorFile=logs/hs_err_pid%p.log, -Xlog:gc*,gc+age=trace,safepoint:file=logs/gc.log:utctime,pid,tags:filecount=32,filesize=64m, -Djava.locale.providers=COMPAT, -XX:UseAVX=2, -Des.cgroups.hierarchy.override=/, -Xms512m, -Xms512m, -Des.path.home=/usr/share/elasticsearch, -Des.path.conf=/usr/share/elasticsearch/config, -Des.distribution.flavor=default, -Des.distribution.type=tar]
[2019-05-17T01:43:21,099][INFO ][o.e.p.PluginsService ] [Ab_wy_n] loaded module [aggs-matrix-stats]
...
[2019-05-17T01:43:29,436][INFO ][o.e.x.s.a.s.FileRolesStore] [Ab_wy_n] parsed [0] roles from file [/usr/share/elasticsearch/config/roles.yml]
[2019-05-17T01:43:30,378][INFO ][o.e.x.m.p.l.CppLogMessageHandler] [Ab_wy_n] [controller/71] [Main.cc@109] controller (64 bit): Version 6.6.0 (Build bbb4919f4d17a5) Copyright (c) 2019 Elasticsearch BV
[2019-05-17T01:43:31,695][INFO ][o.e.d.DiscoveryModule ] [Ab_wy_n] using discovery type [zen] and host providers [settings]
...
[2019-05-17T01:43:33,727][INFO ][o.e.b.BootstrapChecks ] [Ab_wy_n] bound or publishing to a non-loopback address, enforcing bootstrap checks
ERROR: [1] bootstrap checks failed
[1]: initial heap size [536870912] not equal to maximum heap size [1073741824]; this can cause resize pauses and prevents mlockall from locking the entire heap
[2019-05-17T01:43:33,763][INFO ][o.e.n.Node ] [Ab_wy_n] stopping ...
[2019-05-17T01:43:33,784][INFO ][o.e.n.Node ] [Ab_wy_n] stopped
[2019-05-17T01:43:33,787][INFO ][o.e.n.Node ] [Ab_wy_n] closing ...
[2019-05-17T01:43:33,816][INFO ][o.e.n.Node ] [Ab_wy_n] closed
[2019-05-17T01:43:33,818][INFO ][o.e.x.m.p.NativeController] [Ab_wy_n] Native controller process has stopped - no new native processes can be started