Dockerfile

# This is how we usually do things.

FROM alpine:latest AS t1
RUN <<EOF
    find /var/cache/apk
    apk add --no-cache build-base
    find /var/cache/apk
EOF

# This won't actually do anything. Cache needs to be enabled.
# OK to be fair it does cache some meta data. Mostly useless
# to cache that becaues it's like 3 MB in size.

FROM alpine:latest AS t2
RUN <<EOF
    find /var/cache/apk
    apk add build-base
    find /var/cache/apk
EOF

# This will use a cache. But it does the opposite of what we want.
# It always downloads the artifacts; and it stores them in the image!

FROM alpine:latest AS t3
RUN <<EOF
    ln -s /var/cache/apk /etc/apk/cache
    find /var/cache/apk
    apk add build-base
    find /var/cache/apk
EOF

# At first, this looks bad.  But look at the size, the cache will not be baked
# into the image.  The cache is only mounted during build and not kept in the
# final image.
#
# ❯ docker images | egrep '^t[1234]'
# t4   latest    b7aa44fd860c   9 seconds ago    190MB
# t3   latest    9bcc3c4829ec   21 seconds ago   257MB
# t2   latest    0a674420af3e   34 seconds ago   193MB
# t1   latest    5cb337865a72   46 seconds ago   190MB

FROM alpine:latest AS t4
RUN --mount=type=cache,target=/var/cache/apk <<EOF
    ln -s /var/cache/apk /etc/apk/cache
    find /var/cache/apk
    apk add build-base
    find /var/cache/apk
EOF

The point here is that you can update your Dockerfile and change the package deps. The cache will still survive. Across all builds not just the same Dockerfile. That will save you quite a bit of download if you have a dedicated build machine for your CI/CD pipelines. More importantly, if you are building on your laptop and you're on a 3G connection this becomes absolutely a must. :-)