The right code should look like this. Please see what you are missing out:
# Create the channel
docker exec peer0.org1.example.com peer channel create -o 
orderer.example.com:7050 -c composerchannel -f 
/etc/hyperledger/configtx/composer-channel.tx
# Join peer0.org1.example.com to the channel.
docker exec -e "CORE_PEER_MSPCONFIGPATH= 
/etc/hyperledger/msp/users/Admin@org1.example.com/msp 
peer0.org1.example.com peer channel join -b composerchannel.block
# Create the channel
docker exec -e "CORE_PEER_MSPCONFIGPATH= 
/etc/hyperledger/msp/users/Admin@org1.example.com/msp" 
peer1.org1.example.com peer channel fetch config -o orderer.example.com:7050 
-c composerchannel
# Join peer1.org1.example.com to the channel.
docker exec -e "CORE_PEER_MSPCONFIGPATH= 
/etc/hyperledger/msp/users/Admin@org1.example.com/msp 
peer1.org1.example.com peer channel join -b composerchannel_config.block