Pull request: all: update go and backend tools

Closes #2576.
Updates #2275.
Updates #2419.
Updates #2443.

Squashed commit of the following:

commit b1a4809ada298d675de12740051ba26fb9945957
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri May 21 14:01:40 2021 +0300

    all: add --local-frontend, upd docker

commit 619ee7c82f27e3405753003dbec556ffb056d025
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 20 15:02:33 2021 +0300

    bamboo-specs: bump docker version

commit 5c2b2fbce80afdcc81fd0cb83674dc3d64facbf1
Merge: 6536b32d 9c60aef6
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu May 20 15:01:47 2021 +0300

    Merge branch 'master' into 2275-upd-go

commit 6536b32dd4580425f7dedde6765463a79b9bd699
Merge: 9bb32bc4 6f7fd33a
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 19 20:38:48 2021 +0300

    Merge branch 'master' into 2275-upd-go

commit 9bb32bc4c0ac0f3a97195adc75359e48c9c58897
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 19 18:48:50 2021 +0300

    all: fix build, imp err handling

commit 6868eac7f7d2980fb706881f53e72afe5f7c3447
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 19 18:09:32 2021 +0300

    all: fix github lint

commit ebbb9c55f32fbd57e34e8b161016aa6b291c097c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed May 19 17:36:56 2021 +0300

    all: update go and backend tools

.gitignore

@@ -4,14 +4,14 @@
 #
 # See https://stackoverflow.com/a/7335487/1892060.
 #
-# Only build, run, and test outputs here.  Sorted.
-*-packr.go
+# Only build, run, and test outputs here.  Sorted.  With negations at the
+# bottom to make sure they take effect.
 *.db
 *.log
 *.snap
 /bin/
-/build/
-/build2/
+/build/*
+/build2/*
 /data/
 /dist/
 /dnsfilter/tests/dnsfilter.TestLotsOfRules*.pprof
@@ -23,3 +23,6 @@ AdGuardHome*
 coverage.txt
 leases.db
 node_modules/
+
+!/build/gitkeep
+!/build2/gitkeep