Upgrade vendored Dockerfile template to buster

jessie was already LTS-EOL'd in 2020-06
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

changelogs/unreleased/update-vendored-dockerfile-templates.yml

+---
+title: Upgrade vendored Dockerfile template to buster
+merge_request: 42169
+author: Takuya Noguchi
+type: other

doc/api/project_templates.md

@@ -110,7 +110,7 @@ Example response (Dockerfile):
 ```json
 {
   "name": "Binary",
-  "content": "# This file is a template, and might need editing before it works on your project.\n# This Dockerfile installs a compiled binary into a bare system.\n# You must either commit your compiled binary into source control (not recommended)\n# or build the binary first as part of a CI/CD pipeline.\n\nFROM buildpack-deps:jessie\n\nWORKDIR /usr/local/bin\n\n# Change `app` to whatever your binary is called\nAdd app .\nCMD [\"./app\"]\n"
+  "content": "# This file is a template, and might need editing before it works on your project.\n# This Dockerfile installs a compiled binary into a bare system.\n# You must either commit your compiled binary into source control (not recommended)\n# or build the binary first as part of a CI/CD pipeline.\n\nFROM buildpack-deps:buster\n\nWORKDIR /usr/local/bin\n\n# Change `app` to whatever your binary is called\nAdd app .\nCMD [\"./app\"]\n"
 }
 ```
 

doc/api/templates/dockerfiles.md

@@ -124,7 +124,7 @@ Example response:
 ```json
 {
   "name": "Binary",
-  "content": "# This file is a template, and might need editing before it works on your project.\n# This Dockerfile installs a compiled binary into a bare system.\n# You must either commit your compiled binary into source control (not recommended)\n# or build the binary first as part of a CI/CD pipeline.\n\nFROM buildpack-deps:jessie\n\nWORKDIR /usr/local/bin\n\n# Change `app` to whatever your binary is called\nAdd app .\nCMD [\"./app\"]\n"
+  "content": "# This file is a template, and might need editing before it works on your project.\n# This Dockerfile installs a compiled binary into a bare system.\n# You must either commit your compiled binary into source control (not recommended)\n# or build the binary first as part of a CI/CD pipeline.\n\nFROM buildpack-deps:buster\n\nWORKDIR /usr/local/bin\n\n# Change `app` to whatever your binary is called\nAdd app .\nCMD [\"./app\"]\n"
 }
 ```
 

vendor/Dockerfile/Binary.Dockerfile

@@ -2,7 +2,7 @@
 # You must either commit your compiled binary into source control (not recommended)
 # or build the binary first as part of a CI/CD pipeline.
 
-FROM buildpack-deps:jessie
+FROM buildpack-deps:buster
 
 WORKDIR /usr/local/bin
 

vendor/Dockerfile/Golang.Dockerfile

@@ -6,7 +6,7 @@ COPY . .
 RUN go-wrapper download
 RUN go build -v
 
-FROM buildpack-deps:jessie
+FROM buildpack-deps:buster
 
 WORKDIR /usr/local/bin