Question:  Let's Rock Again! is a music documentary film following a singer that was the lead vocalist of what punk band?
Context: Grayscale are an American pop punk band from Philadelphia, Pennsylvania.  Formed in 2011, the group features lead vocalist Collin Walsh, lead guitarist and backup vocalist Andrew Kyne, rhythm guitarist and backup vocalist Dallas Molster, bassist Nick Ventimiglia and drummer Nick Veno.  The band formed from the Philadelphia pop punk scene, building a strong reputation and gaining steam among fans and music lovers.  In April 2015, Grayscale released their first EP "Change" gaining much success.  They are currently signed to Fearless Records. The Way is an American Christian hardcore and Christian rock band, and they primarily play hardcore punk, punk rock, and alternative rock.  They come from the cities of Oxnard, California and Santa Paula, California.  The band started making music in 2010, and their members are lead vocalist, Johnny, lead guitarist and background vocalist, Manny, bassist, Tank, rhythm guitarist and background vocalist, Harry, and drummer, Ryan.  The band have released one extended play, "Helpless but Not Hopeless", in 2011, with Thumper Punk Records.  Their first studio album, "The Fight Is Ours", was released in 2010 by Thumper Punk Records.  The subsequent studio album, "I Keep Falling", was released by Thumper Punk Records, in 2013. A Common Goal is an American Christian punk band, and they primarily play punk rock and alternative rock.  They come from Boston, Massachusetts.  The band started making music in 2009, and their members are lead vocalist and guitarist, Mike May, lead guitarist and background vocalist, Scott Grenon, bassist and background vocalist, Brett Ennis, and drummer and background vocalist, Jason Autrey.  The band have released one extended play, "For God and Country", in 2013, with Thumper Punk Records.  Their first studio album, "Blessings and Battles", was released in 2013 by Thumper Punk Records. Let's Rock Again!  is a music documentary film following Joe Strummer as he tours across the United States and Japan with his band the Mescaleros promoting their second album Global a Go-Go.  The memoir was shot by filmmaker and longtime Strummer friend Dick Rude in the 18 months leading up to Strummer's death in 2002. John Graham Mellor (21 August 1952 – 22 December 2002), known by his stage name Joe Strummer, was an English musician, singer, actor and songwriter who was the co-founder, lyricist, rhythm guitarist and lead vocalist of the Clash, a punk rock band formed in 1976 as part of the original wave of British punk. Steve Haley is a singer, guitarist and songwriter from Philadelphia, Pennsylvania.  He is also a founding member of the trash/punk band Trashlight Vision which he formed with Acey Slade, (Murderdolls, Joan Jett & the Blackhearts).  Haley was also the founding member, lead vocalist and guitarist in the punk rock band Nympho Clepto.  He has toured and shared the stage with international acts such as The New York Dolls, Nazareth, Michael Graves from The Misfits and punk rock legends The Buzzcocks. Clinton Bradley David Bell (born March 18, 1983 in Davison, Michigan), known professionally as Bradley Bell, is an American keyboardist, pianist, synthesist, and backing vocalist, best known for being the keyboardist of post-hardcore band Chiodos, and also for being the keyboardist of pop punk band Cinematic Sunrise until their demise, and the keyboardist of experimental band The Sound of Animals Fighting.  He was also in a pop punk band called Still No Sign, where he was the lead vocalist.  Chiodos guitarist Pat McManaman was also in this band. False Idle is an American Christian hardcore band, and they primarily play hardcore punk and punk rock.  They come from Boise, Idaho.  The band started making music in 2010, and their members are lead vocalist and guitarist, Sef Idle, lead guitarist and background vocalist, Tyler Lewis, bassist and background vocalist, Seth Warren, and drummer and background vocalist, Phil Harris.  The band has released four extended plays, "Hymns of Punk Rock Praise", in 2010, "I Refuse", in 2011, "California or Bust", in 2013, and a split EP with the band, A Common Goal , "Split Decision", in 2014, all with Thumper Punk Records.  Their first full-length studio album, "Threat", was released in 2013 by Thumper Punk Records. Quiet Riot: Well Now You're Here, There's No Way Back is a 2015 documentary film about Los Angeles-based heavy metal and hair metal band Quiet Riot.  The film was directed by former actress Regina Russell Banali.  It premiered January 29, 2015, on Showtime and was screened out of competition at the 2015 Cannes Film Festival.  The film had its festival premier at the 2014 Newport Beach Film Festival and won the Festival Honors award for "Outstanding Achievement in Filmmaking" in the music category.  It also won "Best Music Documentary" from the Oregon Independent Film Festival, and was an official selection of The Hollywood Film Festival, the Carmel International Film Festival, Indie Memphis Film Festival, St. Louis International Film Festival, and Big Sky Documentary Film Festival. The Finger was a hardcore punk band, formed by Ryan Adams and Jesse Malin, under the pseudonyms "Warren Peace" and "Irving Plaza" respectively (along with Colin Burns and Johnny T. Yerington as "Jim Beahm" and "Rick O'Shea").  The name derived from notorious early/mid-1990s Raleigh, North Carolina rock band Finger, of which Adams was a big fan.  This light-hearted project allowed both artists to return to their punk backgrounds (Adams began his music career as singer for The Patty Duke Syndrome and Malin began his career in the hardcore punk band Heart Attack and more famously as the lead singer of D Generation).  They began by releasing two EPs: "We Are Fuck You" and "Punk's Dead Let's Fuck" which were later collected to form the album "We Are Fuck You", released in 2003.
Answer: the Clash