Dockerfile(.simple): align APT_MIRROR support

Use a non-slash escape sequence to support mirrors with a path
component, and do not unconditionally replace the mirror in
Dockerfile.simple.

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

Dockerfile

@@ -34,7 +34,7 @@ FROM --platform=$BUILDPLATFORM ${GOLANG_IMAGE} AS base
 COPY --from=xx / /
 RUN echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
 ARG APT_MIRROR
-RUN test -n "$APT_MIRROR" && sed -ri "s/(httpredir|deb|security).debian.org/${APT_MIRROR}/g" /etc/apt/sources.list || true
+RUN test -n "$APT_MIRROR" && sed -ri "s#(httpredir|deb|security).debian.org#${APT_MIRROR}#g" /etc/apt/sources.list || true
 ARG DEBIAN_FRONTEND
 RUN apt-get update && apt-get install --no-install-recommends -y file
 ENV GO111MODULE=off

Dockerfile.simple

@@ -13,9 +13,9 @@ ARG GOLANG_IMAGE="golang:${GO_VERSION}-${BASE_DEBIAN_DISTRO}"
 FROM ${GOLANG_IMAGE}
 ENV GO111MODULE=off
 
-# allow replacing httpredir or deb mirror
-ARG APT_MIRROR=deb.debian.org
-RUN sed -ri "s/(httpredir|deb).debian.org/$APT_MIRROR/g" /etc/apt/sources.list
+# allow replacing debian mirror
+ARG APT_MIRROR
+RUN test -n "$APT_MIRROR" && sed -ri "s#(httpredir|deb|security).debian.org#${APT_MIRROR}#g" /etc/apt/sources.list || true
 
 # Compile and runtime deps
 # https://github.com/docker/docker/blob/master/project/PACKAGERS.md#build-dependencies