Course I · Docker
+An image is not a box.
It is a stack of frozen diffs.
+ Scroll, and the thing you keep calling "a container image" comes apart in your + hands. Four layers. Each one only stores what changed from the layer under it - and here they + rise one at a time, bottom up.
+A stack of diffs wearing a name
+An image is not a copy of a machine. It is a stack of read-only layers, each recording only + what changed from the one beneath. The base OS is pulled once and shared by every image on + the host; your dependencies sit on it; your code - usually the smallest layer, always the + most volatile - sits above that; and the writable layer is created fresh per container and + thrown away when it dies. Nothing you write there survives.
+Change a layer and every layer above it must be rebuilt.
+The order is a caching decision
+The builder caches layer by layer, and a cached layer survives only while everything + beneath it is unchanged. Put COPY . . above your dependency + install and you have told the builder to throw the dependency cache away every time one + line of code changes. Dependencies first, code last - a Dockerfile is a cache policy that + happens to build software.
+Field note. A rebuild that takes twenty minutes and one that + takes twenty seconds are usually the same Dockerfile with the lines swapped.
+This stack explains the whole ecosystem above it: sharing explains why + pulls are fast, immutability explains why a digest can name the exact bytes (Course VII), + and the throwaway top layer explains why state needs volumes. One idea - frozen diffs - + all the way down.
+Pre-reads: none - start here + Further: Docker get-started · + Docker build + layers
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