refactor: move all shellcheck disable definitions to .shellcheckrc file

This makes standard use of shellcheck work without needing to provide extra configuration anywhere.

Also remove use of inline 'shellcheck disable' calls that are already defined in the .shellcheckrc and don't need to be set inline.

debian/config

@@ -3,7 +3,6 @@ set -eo pipefail
 [[ $TRACE ]] && set -x
 
 if [[ -e /usr/share/debconf/confmodule ]]; then
-  # shellcheck disable=SC1091
   . /usr/share/debconf/confmodule
 fi
 

debian/postinst

@@ -3,12 +3,10 @@ set -eo pipefail
 [[ $TRACE ]] && set -x
 
 if [[ -e /usr/share/debconf/confmodule ]]; then
-  # shellcheck disable=SC1091
   . /usr/share/debconf/confmodule
 fi
 
 if [[ -r /etc/default/dokku ]]; then
-  # shellcheck disable=SC1091
   source /etc/default/dokku
 fi
 

debian/postrm

@@ -3,7 +3,6 @@ set -eo pipefail
 [[ $TRACE ]] && set -x
 
 if [[ -e /usr/share/debconf/confmodule ]]; then
-  # shellcheck disable=SC1091
   . /usr/share/debconf/confmodule
 fi
 

debian/preinst

@@ -3,7 +3,6 @@ set -eo pipefail
 [[ $TRACE ]] && set -x
 
 if [[ -e /usr/share/debconf/confmodule ]]; then
-  # shellcheck disable=SC1091
   . /usr/share/debconf/confmodule
 fi