add NETRC_FILE to allow passing credentials into kas home

Tools like wget and git can read credentials from $HOME/.netrc for
servers that require authentication. Allow users to pass in a .netrc
file into the kas home dir to support i.e. bitbake https fetching with
auth.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
[Jan: style fix in command-line.rst]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas-container

@@ -435,6 +435,11 @@ if [ -n "${GIT_CREDENTIAL_HELPER}" ] ; then
 	set -- "$@" -e GIT_CREDENTIAL_HELPER="${GIT_CREDENTIAL_HELPER}"
 fi
 
+if [ -f "${NETRC_FILE}" ]; then
+	set -- "$@" -v "$(readlink -f "${NETRC_FILE}")":/etc/skel/.netrc:ro \
+		-e NETRC_FILE="/etc/skel/.netrc"
+fi
+
 if [ -t 1 ]; then
 	set -- "$@" -t -i
 fi