escape the sed replacements, and stop the script failing silently

The sed pass that configures the daemon punished anyone setting a strong
password: '&' in a replacement means the whole match, so a password
containing one was silently rewritten into something else, and the '|'
delimiter made sed exit with 'unknown option to s'. Every setting rode in
one invocation and the script had no set -e, so that failure applied none
of them -- not apipassword, not apienabled, not apiinterface -- and the
daemon came up on whatever it had before without saying so.

esc() now escapes backslash, '&' and the delimiter via printf. The
expressions are anchored to the start of the line and name their key in the
replacement instead of using \1, so no backreference is involved and
nothing in another section can match. keys.dat also holds privsigningkey
and privencryptionkey for this node's Bitmessage identities; editing in
place leaves every byte outside [bitmessagesettings] untouched, which is
why this stays a line edit rather than a parse-and-rewrite.

The clients needed the other half of this: credentials go into an XML-RPC
URL, where '@' splits the userinfo and '#' truncates the rest, so a strong
password wrote correctly and still failed to connect. Both now
percent-encode. A '%' in the password remains unusable -- it breaks
PyBitmessage's own config reader and every API call returns 500.

Also here, all found while making the above safe:
- set -eu, with the keys.dat chown guarded. A misconfiguration now stops
  the container instead of passing unnoticed.
- The seed-address retry loop used bash brace expansion under CMD ["sh"],
  where /bin/sh is dash and {1..4} is a literal, so it ran once, not four
  times.
- apt-get update shared a layer with nothing, letting a cached update feed
  install months-stale package lists.
- HEALTHCHECK gained a start period; the startup VACUUM takes tens of
  seconds on a large messages.dat and the container reported unhealthy for
  all of it.
- Removed the AppImage systemd unit, AppArmor profile and updater script.
  Nothing referenced them -- not the image, the deployment or the ansible
  roles -- and the updater fetched a binary with no signature or checksum
  check.

docker/Dockerfile

@@ -3,10 +3,11 @@ FROM ubuntu:bionic
 
 SHELL ["/bin/bash", "-exo", "pipefail", "-c"]
 
-RUN apt-get update
-
-# Install dependencies
-RUN apt-get install -yq --no-install-suggests --no-install-recommends \
+# Install dependencies. update and install share a layer on purpose: split
+# across two, a cached update feeds install package lists that may be months
+# stale, and the install then fails or pulls something unintended.
+RUN apt-get update \
+ && apt-get install -yq --no-install-suggests --no-install-recommends \
     build-essential libcap-dev libssl-dev \
     python-all-dev python-msgpack python-pip python-setuptools \
     git
@@ -72,7 +73,10 @@ RUN su bitmessage -c "pybitmessage -t"
 
 CMD ["sh", "/usr/local/bin/run.sh"]
 
-## Check PyBitmessage active network connections
-HEALTHCHECK --retries=0 --interval=15s \
+## Check PyBitmessage active network connections.
+## The start period covers the startup VACUUM of messages.dat, which takes tens
+## of seconds once the database reaches a few hundred MB; without it the
+## container reports unhealthy for that whole window on every restart.
+HEALTHCHECK --retries=0 --interval=15s --start-period=180s \
   CMD ["python", "/usr/local/bin/healthy_check.py"]