Default worker-count in Docker: 1 instead of 10

My reasoning is twofold:

1. a count of 1 is actually probably "enough for many setups" because ingestion
   is very fast anyway (just store the file); while the lower memory-footprint
   that comes with having fewer workers means that adoption of Bugsink will be
   easier.

2. tuning the variable is going to be less annoying if there's no default that
   might override "in the wrong direction" (i.e. the interaction between
   `WEB_CONCURRENCY` and `GUNICORN_CMD_ARGS`)

See #101 (where the concept of customizability was discussed, though not the
present idea of chaning the default).

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Changes
 
+## x.x.x (xx xxx xxxx)
+
+### Backwards-incompatible changes
+
+* The default number of web processes (gunicorn server workers) in the
+  dockerized setup is now 1 (it used to be 10).
+
+  set `WEB_CONCURRENCY=...` or `GUNICORN_CMD_ARGS="--workers=..."` to
+  restore the previous behavior or choose a custom number.
+
+### ...
+
 ## 1.5.4 (12 May 2025)
 
 * Add bugsink-util script to allow settings-independent commands to be run