Compose: Interpretação inconsistente do ambiente em docker-compose

Se eu executar a imagem oficial do tomcat com este comando:

docker exec -d -e JAVA_OPTS = "- Xms5000m -Xmx10000m" tomcat: 8
tomcat começa com sucesso

mas se eu criar docker-compose.yml com este conteúdo

tomcat:
  image: tomcat:8
  environment:
    - JAVA_OPTS="-Xms5000m -Xmx10000m"

e então eu corro:

docker-compose up

Eu recebo esta saída:

Recreating temp_tomcat_1...
Attaching to temp_tomcat_1
tomcat_1 | Invalid initial heap size: -Xms5000m -Xmx10000m
tomcat_1 | Error: Could not create the Java Virtual Machine.
tomcat_1 | Error: A fatal exception has occurred. Program will exit.
temp_tomcat_1 exited with code 1
Gracefully stopping... (press Ctrl+C again to force)

Alguma ideia do que está errado?

meu sistema:

docker -v
Docker version 1.5.0, build a8a31ef/1.5.0
docker-compose --version
docker-compose 1.1.0
 uname -a
Linux docker.mzk.cz 3.18.7-200.fc21.x86_64 #1 SMP Wed Feb 11 21:53:17 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
docker -D info
Containers: 11
Images: 40
Storage Driver: btrfs
 Build Version: Btrfs v3.18.1
 Library Version: 101
Execution Driver: native-0.2
Kernel Version: 3.18.7-200.fc21.x86_64
Operating System: Fedora 21 (Twenty One)
CPUs: 2
Total Memory: 15.67 GiB
Name: docker.mzk.cz
ID: GYRC:LL2X:GT6M:IAGQ:UUIN:XQ7Q:WHUY:SICW:IB3Z:FJZP:LCDI:K6LW
Debug mode (server): false
Debug mode (client): true
Fds: 22
Goroutines: 40
EventsListeners: 0
Init SHA1: a2b40aadd44cc16541a4c34c5572d145d2c052d5
Init Path: /usr/libexec/docker/dockerinit
Docker Root Dir: /var/lib/docker