Drastically reduce the main image size

Merged Georges Racinet requested to merge topic/default/main-image-size into branch/default

This is #5 (closed) (which saves us about a whooping GB) followed by more actions: reducing the layers and being more careful with apt and pip.

Here's the result I get:

~/heptapod/docker $ docker image list octobus/heptapod | head -3 
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
octobus/heptapod    testing             5c4d9aadd23b        15 minutes ago      2.02GB
octobus/heptapod    0-12-testing        387afcb08a8a        47 hours ago        3.11GB

An interesting metric is the comparison with GitLab base image:

~/heptapod/docker $ docker image list gitlab/gitlab-ce | head -3
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
gitlab/gitlab-ce    12.2.12-ce.0        1fe894d81d4f        4 months ago        1.76GB
gitlab/gitlab-ce    12.3.5-ce.0         7783a0d67474        6 months ago        1.77GB

These don't change in the lifetime of a Heptapod version, from 0.x.0rc1 to 0.x.y.

In particular, retrieving a freshly built image with my lousy DSL line now completes in less than 2 minutes, given that the GitLab base image is already present.

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