add docker image release for Ubuntu based image

ci/release-image/Dockerfile

@@ -1,9 +1,10 @@
 # syntax=docker/dockerfile:experimental
 
+ARG BASE=debian:11
 FROM scratch AS packages
 COPY release-packages/code-server*.deb /tmp/
 
-FROM debian:11
+FROM $BASE
 
 RUN apt-get update \
   && apt-get install -y \

ci/release-image/docker-bake.hcl

@@ -6,17 +6,63 @@ variable "VERSION" {
     default = "latest"
 }
 
+variable "DOCKER_REGISTRY" {
+    default = "docker.io/codercom/code-server"
+}
+
+variable "GITHUB_REGISTRY" {
+    default = "ghcr.io/coder/code-server"
+}
+
 group "default" {
-    targets = ["code-server"]
+    targets = [
+        "code-server-debian-11", 
+        "code-server-ubuntu-focal",
+    ]
+}
+
+function "prepend_hyphen_if_not_null" {
+    params = [tag]
+    result = notequal("","${tag}") ? "-${tag}" : "${tag}"
+}
+
+# use empty tag (tag="") to generate default tags
+function "gen_tags" {
+    params = [registry, tag]
+    result = notequal("","${registry}") ? [
+        notequal("", "${tag}") ? "${registry}:${tag}" : "${registry}:latest",
+        notequal("latest",VERSION) ? "${registry}:${VERSION}${prepend_hyphen_if_not_null(tag)}" : "",
+    ] : []
+}
+
+# helper function to generate tags for docker registry and github registry.
+# set (DOCKER|GITHUB)_REGISTRY="" to disable corresponding registry
+function "gen_tags_for_docker_and_ghcr" {
+    params = [tag]
+    result = concat(
+        gen_tags("${DOCKER_REGISTRY}", "${tag}"),
+        gen_tags("${GITHUB_REGISTRY}", "${tag}"),
+    )
 }
 
-target "code-server" {
+target "code-server-debian-11" {
     dockerfile = "ci/release-image/Dockerfile"
-    tags = [
-        "docker.io/codercom/code-server:latest",
-        notequal("latest",VERSION) ? "docker.io/codercom/code-server:${VERSION}" : "",
-        "ghcr.io/coder/code-server:latest",
-        notequal("latest",VERSION) ? "ghcr.io/coder/code-server:${VERSION}" : "",
-    ]
+    tags = concat(
+        gen_tags_for_docker_and_ghcr(""),
+        gen_tags_for_docker_and_ghcr("debian"),
+        gen_tags_for_docker_and_ghcr("bullseye"),
+    )
+    platforms = ["linux/amd64", "linux/arm64"]
+}
+
+target "code-server-ubuntu-focal" {
+    dockerfile = "ci/release-image/Dockerfile"
+    tags = concat(
+        gen_tags_for_docker_and_ghcr("ubuntu"),
+        gen_tags_for_docker_and_ghcr("focal"),
+    )
+    args = {
+        BASE = "ubuntu:focal"
+    }
     platforms = ["linux/amd64", "linux/arm64"]
 }