How to fetch a image with proxy?

[ghost]

Hello everyone!
I want to use a proxy server when I fetch a image , but I don't know how to do that.

rkt Version: 1.30.0
appc Version: 0.8.11
Go Version: go1.8.3
Go OS/Arch: linux/amd64
Features: -TPM +SDJOURNAL
--
Linux 4.15.0-33-generic x86_64
--
NAME="Ubuntu"
VERSION="18.04.1 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.1 LTS"
VERSION_ID="18.04"
......
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic
--
systemd 237
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN -PCRE2 default-hierarchy=hybrid

What did I do?
I try to export a variable (e.g. export HTTP_PROXY='https://localhost:xxxx'), but it still not any infomation about proxy in the output of rkt fetch ... , look like it's doesn't work. Who can help me? 😞