add --channel option

Add a --channel option as alternative to the $CHANNEL environment variable;

    ./install.sh --channel test --dry-run
    # Executing docker install script, commit:
    apt-get update -qq >/dev/null
    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq apt-transport-https ca-certificates curl gnupg >/dev/null
    install -m 0755 -d /etc/apt/keyrings
    curl -fsSL "https://download.docker.com/linux/ubuntu/gpg" | gpg --dearmor --yes -o /etc/apt/keyrings/docker.gpg
    chmod a+r /etc/apt/keyrings/docker.gpg
    echo "deb [arch=arm64 signed-by=/etc/apt/keyrings/docker.gpg] https://download.docker.com/linux/ubuntu jammy test" > /etc/apt/sources.list.d/docker.list
    apt-get update -qq >/dev/null
    DEBIAN_FRONTEND=noninteractive apt-get install -y -qq docker-ce docker-ce-cli containerd.io docker-compose-plugin docker-ce-rootless-extras docker-buildx-plugin >/dev/null

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

install.sh

@@ -48,6 +48,10 @@ mirror=''
 DRY_RUN=${DRY_RUN:-}
 while [ $# -gt 0 ]; do
 	case "$1" in
+		--channel)
+			CHANNEL="$2"
+			shift
+			;;
 		--dry-run)
 			DRY_RUN=1
 			;;