Protect against inconsistent use of / separators in APT paths

As a matter of fact, our own fallback would not even have worked :-(
The `readlink` invocations clean up duplicated and trailing slashes.

scripts-slim/tweak-apt-config

@@ -5,14 +5,14 @@
 # License: GPL-3.0+
 
 DIR=/
-ETC=etc/apt
+ETC=etc/apt/
 PARTS=apt.conf.d
 
 eval $(apt-config shell DIR Dir)
 eval $(apt-config shell ETC Dir::Etc)
 eval $(apt-config shell PARTS Dir::Etc::Parts)
 
-cfg=$DIR$ETC$PARTS
+cfg=$(readlink -f /$DIR/$ETC/$PARTS)
 
 cat > $cfg/docker-recommends <<EOF
 APT::Install-Recommends "false";

scripts/docker-apt-clean

@@ -14,5 +14,7 @@ CACHE=var/cache/apt/
 eval $(apt-config shell DIR Dir)
 eval $(apt-config shell CACHE Dir::Cache)
 
-find $DIR$CACHE -type f -name '*.deb' -delete
-find $DIR$CACHE -type f -name '*.bin' -delete
+cache=$(readlink -f /$DIR/$CACHE)
+
+find $cache -type f -name '*.deb' -delete
+find $cache -type f -name '*.bin' -delete

scripts/remove-apt-lists

@@ -14,4 +14,6 @@ STATE=var/lib/apt/
 eval $(apt-config shell DIR Dir)
 eval $(apt-config shell STATE Dir::State)
 
-find $DIR$STATE -type f -name '*_dists_*' -delete
+state=$(readlink -f /$DIR/$STATE)
+
+find $state -type f -name '*_dists_*' -delete

scripts/tweak-apt-config

@@ -5,14 +5,14 @@
 # License: GPL-3.0+
 
 DIR=/
-ETC=etc/apt
+ETC=etc/apt/
 PARTS=apt.conf.d
 
 eval $(apt-config shell DIR Dir)
 eval $(apt-config shell ETC Dir::Etc)
 eval $(apt-config shell PARTS Dir::Etc::Parts)
 
-cfg=$DIR$ETC$PARTS
+cfg=$(readlink -f /$DIR/$ETC/$PARTS)
 
 cmd=$(dirname $0)/docker-apt-clean
 cat > $cfg/docker-clean <<EOF