Dockerfile: Use APT_MIRROR for security.debian.org as well

The fastly cdn mirror we're using also mirrors the debian security
repository;

```
Welcome to deb.debian.org (fastly instance)!

This is deb.debian.org. This service provides mirrors for the following Debian archive repositories:

/debian/
/debian-debug/
/debian-ports/
/debian-security/
The server deb.debian.org does not have packages itself, but the name has SRV records in DNS that let apt in stretch and later find places.
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -27,9 +27,9 @@
 ARG CROSS="false"
 
 FROM golang:1.12.7 AS base
-# allow replacing httpredir or deb mirror
-ARG APT_MIRROR=deb.debian.org
-RUN sed -ri "s/(httpredir|deb).debian.org/$APT_MIRROR/g" /etc/apt/sources.list
+ARG APT_MIRROR
+RUN sed -ri "s/(httpredir|deb).debian.org/${APT_MIRROR:-deb.debian.org}/g" /etc/apt/sources.list \
+ && sed -ri "s/(security).debian.org/${APT_MIRROR:-security.debian.org}/g" /etc/apt/sources.list
 
 FROM base AS criu
 # Install CRIU for checkpoint/restore support