It's simple env based config file transform for docker boot time configuration.
You can download binary
$cat a.conf
[servera]
HOST={{ .HOST | default "B" }}
$export HOST="HOSTA"
$envconf a.conf
$cat a.conf
[servera]
HOST=HOSTA
$envconf a.conf b.conf ...
or
$export ENV_CONF_FILES=a.conf:b.conf:c.conf
$envconf