Docker hub image fails but building its Dockerfile works. What is happening?

Question

I have used Docker-compose a lot recently, but this time I found a container I really want to use but the docker hub’s image is not compatible with my arm/v6 raspberry pi. Using it anyway results in

standard_init_linux.go:219: exec user process caused: exec format error

Strangely, copying the Dockerfile and building it with

    build:
      context: ./ttrss-docker/src/app

results in the app working well. But for some reason, I can’t use the dockerhub’s image.

In case it matters, the Dockerfile is this, and the Docker Hub image is this.

FROM alpine:3.12
EXPOSE 9000/tcp

RUN apk add --no-cache dcron php7 php7-fpm \
    php7-pdo php7-gd php7-pgsql php7-pdo_pgsql php7-mbstring \
    php7-intl php7-xml php7-curl php7-session \
    php7-dom php7-fileinfo php7-json \
    php7-pcntl php7-posix php7-zip php7-openssl \
    git postgresql-client sudo

ADD startup.sh /
ADD updater.sh /
ADD index.php /
ADD dcron.sh /
ADD backup.sh /etc/periodic/weekly/backup

RUN sed -i.bak 's/^listen = 127.0.0.1:9000/listen = 9000/' /etc/php7/php-fpm.d/www.conf
RUN sed -i.bak 's/\(memory_limit =\) 128M/\1 256M/' /etc/php7/php.ini

RUN mkdir -p /var/www

CMD /startup.sh

Question: if I don’t use the Docker hubs image, can Watchtower update my container ? If not, does anyone know what’s happening and how I can achieve a container that updates via Watchtower ?

Many thanks :)

Answer 1

The image you are pulling has only been built for a single architecture: amd64. The resulting binaries and libraries are not usable on other platforms like ARM used by the Raspberry Pi. Below are the debugging steps to verify this.

The manifest is application/vnd.docker.distribution.manifest.v2+json:

$ regctl image manifest --list cthulhoo/ttrss-fpm-pgsql-static
{                                                                                        
  "schemaVersion": 2,
  "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
  "config": {
    "mediaType": "application/vnd.docker.container.image.v1+json",
    "size": 4257,                                                      
    "digest": "sha256:916ae5126809992b922c5db0f41e62a40be245703685e19f51797db95f312e81"
  },
  ...

Checking the architecture of that image:

$ regctl image inspect cthulhoo/ttrss-fpm-pgsql-static --format '{{.Architecture}}'                                    
amd64 

This would need to be fixed by the image creator to build an image for ARM platforms, which you see with the Alpine base image.

$ regctl image manifest --list alpine:3.12        
{                                                          
  "schemaVersion": 2,    
  "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
  "manifests": [
    {                                             
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 528,       
      "digest": "sha256:074d3636ebda6dd446d0d00304c4454f468237fdacf08fb0eeac90bdbfa1bac7",
      "platform": {
        "architecture": "amd64",                 
        "os": "linux"                                                                                                                                                                                                                                                           
      }           
    },
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 528,
      "digest": "sha256:096ebf69d65b5dcb3756fcfb053e6031a3935542f20cd7a8b7c59e1b3cb71558",
      "platform": {
        "architecture": "arm",
        "os": "linux",
        "variant": "v6"
      }
    },
    {
      "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
      "size": 528,
      "digest": "sha256:299294be8699c1b323c137f972fd0aa5eaa4b95489c213091dcf46ef39b6c810",
      "platform": {
        "architecture": "arm",
        "os": "linux",
        "variant": "v7"
      }
    },
    ...

Building multi-platform images is often done with buildx. The regctl command used above is part of my regclient project.