Anchor VERSION match on the Docker epoch to avoid distro-version hits

[fudianchn]

Problem

On Ubuntu noble, VERSION=24.0 installs the latest Docker (28.4.0) instead of the requested 24.0 (#526):

INFO: apt-cache madison docker-ce | grep '24.0' | head -1 | ...

The grep matches VERSION anywhere in the madison line, so 24.0 matches the 24.04 in 5:28.4.0-1~ubuntu.24.04~noble, and head -1 then resolves to 28.4.0.

Fix

Anchor the grep on the madison version field, with the Docker epoch (5:) optional:

- search_command="apt-cache madison docker-ce | grep '$pkg_pattern' | head -1 | ..."
+ search_command="apt-cache madison docker-ce | grep -E ' \\| (5:)?$pkg_pattern' | head -1 | ..."

Applied to both the docker-ce and docker-ce-cli lookups. The version field always starts right after the | separator, so the pattern binds to the start of the package version and cannot match the distro version embedded in the release suffix. The epoch is optional because deb packages before 18.03 (17.x) were published without the 5: epoch, and those pins keep working.

Verification

Simulated apt-cache madison docker-ce output (noble) with both 28.4.0 and 24.0.2 present, VERSION=24.0:

grep	selected (cut -f3)
grep '24.0' (before)	5:28.4.0-1~ubuntu.24.04~noble (wrong: matched 24.04)
grep -E ' \| (5:)?24.0' (after)	5:24.0.9-1~ubuntu.24.04~noble (correct)
grep -E ' \| (5:)?17.03' (after, pre-epoch package)	17.03.2~ce-0~ubuntu-xenial (correct)

And when 24.0 is genuinely unavailable, the anchored grep matches nothing -> the script hits its existing ERROR: '$VERSION' not found amongst apt-cache madison results path and exits 1, instead of silently installing the latest.

bash -n install.sh passes; the matching table above is produced by running the exact pipeline from install.sh against the simulated madison output. The repository CI (including shellcheck) has not run on this fork PR yet and will execute once approved.

Closes #526

[742764146]

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